My Little Pony: Friendship Is Magic is THE biggest example of this phenomenon today. A show about magical cartoon ponies that go on adventures and learn about friendship has more teenaged and adult fans than child fans. While there are tons of shows with unlikely fandoms (some of which are listed here), you'd be hard pressed to find a Periphery Demographic more well known or vocal than the Bronies. Even the writers, animators, and staff are aware of how prominent they are and have even had episodes, jokes, and characters that cater to them.
Sadly, despite the large demographic, this hasn't prevented male viewers open about their love of the show from being bullied for liking a "girl's show", as when an 11-year old boy tried to commit suicide when other students harassed him and called him gay because of his open passion for the program.
Cartoon Network is all about the Periphery Demographic. Recognizing this was what led them to create [adult swim] in 2001. Most Cartoon Network originals have scored a healthy Periphery Demographic, but just to name a few:
Ed, Edd n Eddy has a large following among kids, teens, and adults as the show is more or less a reflection of the audience's real-world childhoods (as the audience probably has dealt with a-hole older brothers, scheming friends, bratty younger siblings, sleazy girls who will stop at nothing to make you their boyfriends/future husbands, Funny Foreigner exchange program kids, and weirdos who have inanimate objects for friends).
The small but vocal Time Squad fandom is mostly composed of teen/20-something-year-old fans who remember the show as children and finally see all the Ho Yay, innuendo, and assorted parental bonuses, or those who were old enough to see the Ho Yay, innuendo, and assorted parental bonuses when it first came on in 2001.
Teen Titans: The show really treads the line on Periphery Demographics, with tongue-in-cheek call outs to comic book plots and characters along the lines of obfuscating Robin's identity via episodes with characters like Red X and Larry, and of course "Control Freak" is pretty much a shout out and/or back-handed compliment to fanboys everywhere.
Young Justice: It's a show about teenagers for a young demographic, but the plotlines and characters are of a quality found in most pay cable dramas. It's a common feature of Greg Weisman shows, see Gargoyles below.
MAD has earned some older fans because of their references to movies and shows targeted towards teenagers and adults, their tendency to make jokes that feel naughtier than expected for children, and their connections to MAD (even though the show is said to be a redone version of the FOX show MADtv).
Courage the Cowardly Dog seemed to have developed a following of teenagers as well as adults, mostly because kids can't watch some of the more scary content stuck in some episodes.
Johnny Bravo: 95% of the humor flies right over the heads of children and/or isn't considered appropriate for kids to start with. Add to that its premise ("A Casanova Wannabe who sounds like Elvis Presley tries and fails at scoring with women") and the fact that this was one of Seth MacFarlane's projects before Family Guy (in fact, Seth did a lot of kids' shows before he was hired by FOX to do Family Guy, American Dad!, and The Cleveland Show) and you start to wonder whether Cartoon Network really intended this to be for kids (they didn't, but this was pre-[adult swim] era Cartoon Network, where the only adult things they had on the channel were old classic cartoon shorts, Space Ghost Coast to Coast, most of the anime they showed at the time note even if it was toned down to kids' show levels, The Flintstonesnote since that was considered the first primetime animated show), and, in its really early days, Wait Till Your Father Gets Home (which, back in the 1970s, was best described as All in the Family if it were animated —- and, these days, is seen as a prototypical version of The Simpsons and/or Family Guy).
For some odd reason, the show also has many fans who are toddlers, due to tons merchandise aimed for toddlers, and the fact that it airs immediately after the Nick Jr. block. Several studies (and Nickelodeon themselves) have proven that the show is not for toddlers.
The Ren & Stimpy Show was originally rated as U (all ages) or TV-Y7 for those in America, but the users of the commonsensemedia.com website rated it to be suitable for viewers above 13 years of age, due to its disturbing imagery (as provided by John Kricfalusi) and gross-out humor. The show is very popular among adult animation nerds.
Avatar: The Last Airbender is a bit of a subversion. The idea was pitched to other networks before Nickelodeon picked it up. It was never exclusively meant for children in the beginning.
The same can be said about Sequel SeriesThe Legend of Korra. To put it in perspective, the 1-hour series premiere had a 1.01 adults 18-49 rating out of its 4.5 million viewers, and another episode had almost 4.1 million total and a 1.13 adults 18-49 rating. Teenagers are also loving it as well, to the point where it's scheduled to start airing on Degrassi-loving TeenNick!
It is worth pointing out that Invader Zim is riddled to the brim with Black Comedy, meaning that the target demographic simply got traumatized.
The Fairly OddParents started out mainly for children of elementary school age, but with its crazy characters, hilarious plots, and countless pop-culture references that children would not understand, it has become a huge hit with adults and adolescents. Creator Butch Hartman points out that a the success of a show intended for children can be measured by its popularity with older viewers.
Iron Man: Armored Adventures has a surprisingly large female fanbase, which is usually unheard of for superhero shows, as well as a decent number of anime fans. This can probably be explained due to the series' heavy anime-influence which is popular with both unintended demographics.
Danny Phantom. Intended for preteen boys, apparently, but the majority of fans tend to be female. And of the college age.
The show also enjoys a strong following amongst college/adult male cartoon fans who seem drawn to the female characters' often buxom designs. And they don't call them Hartman Hips for nothing!
Doug was rather popular with adults during its run on both Nick and ABC, mainly from parents who wanted their kids to watch it.
KaBlam! kind of inverts this. It had a pretty big audience of the usual seven to eleven demographic, though earlier, Nick was trying to target the show towards teenagers (while still keeping it TV-Y7). The show was also had a solid adult fanbase as well.
Teenage Mutant Ninja Turtles (2012) has gained an audience in this manner fairly quickly. Much like the Power Rangers Samurai example in Live Action TV, the show brought back the original theme song and Rob Paulsen in a voice acting role as one of the Turtles; and the writers and animators have made the show more in line with the campy and colorful 1987 show. It also has gained a young adult and adult audience through its distinct take on the Turtles' origin story.
The intended audience of Totally Spies! was intended to be elementary and middle school age girls, but due to the show's fetishistic overtones, the show has found its primary audience among college age males.
Lots of people love Kim Possible, many of them parents themselves.
This is evidenced by its strong showing on fan-themed sites such as Fanfiction.net where, as of October 2010, it registered over 7,000 posted stories, one of the largest listings in the Cartoon category. (A good proportion of them being Les Yayslash fics. And the writers are well aware of the popularity of these pairings.)
also this show is heavily prone to Rule 34, many of them Kim/Shego themed.
As well as by Disney's 2007 "Everything Kim Possible" marathon aired over several days virtually non-stop, well outside its intended "tween/teen girl" demographic hours.
The Avengers: Earth's Mightiest Heroes has a target audience of 9-14 year old boys, but has its fair share of adult fans as well. These include Marvel Comics readers who loved finding tough and relatively deep portrayals of their favorite characters, and even some likable re-interpretations of characters they had lukewarm or negative feelings about while reading the comics. The dynamic and well-animated action sequences also helped it earn older fans. Some people say this show boasts as many universally appeaing qualities as shows from the DCAU.
The periphery demographic ultimately came back to bite the show in the ass. While it maintained relatively high ratings, most of the viewers were outside the show's target demo. Comparatively, the Lighter and SofterUltimate Spider-Man continues to be a Base Breaker online, but did much better among the network's target demographic. As such, Earth's Mightiest Heroes has been cancelled, with Avengers Assemble taking its place.
Similarly, The Spectacular Spider-Man was aimed at young and pre-teen boys, but is quite popular with teens and adults, given its great character reinterprataions and development, awesome script and voice acting, wonderful tributes to classic Spidey stories and lore, and the surprisingly large amount of Getting Crap Past the Radar. It even holds a good following after its premature cancelation.
Really, most animated shows based off superhero comics end up getting a lot of older viewers, as often they follow the comics in having good characters and story arcs. The first X-Men animated series was a similar case, as was X-Men: Evolution. (it's often stated that out of the superhero animated shows, Evolution was the most popular with teenage girls) Spider-Man: The Animated Series was the same way.
As of August 1, 2012, Wild Kratts had approximately 200 listed tropes on its TV Tropes page, 80 fanfictions on Fanfiction.net, and roughly 57 members in the #Team-Wild Kratt group on deviantART. That may seem small, but when you consider the fact that the show is marketed towards eight-year-olds and that some of the fanart and fanfictions rival the actual show in quality, you start to wonder.
It's worth noting that there's a fandom on Tumblr, with too many members to count (myself being one), that pretty much grows daily. Heck, there's even a fan-made episode currently in production!
This particular dinosaur enthusiast actually learned far more about dinosaurs watching this than I did in school.
Peep And The Big Wide World is meant for preschoolers, but is well-liked by older audiences due to funny moments, good writing, and interesting characters.
Near the end of Bugs Bunny and Tweety's run on ABC, the show was still receiving a respectable total number of viewers, mainly outside the 2-11 audience. It was only cancelled because their contract ran up, Cartoon Network was airing most of the cartoons The Bugs Bunny and Tweety Show was airing (with most offending scenes shown uncut and uncensored), and ABC by then was airing mostly Disney-based shows.
This was largely the downfall of many WB Saturday morning cartoons, including Freakazoid!: While older audiences would frequently tune in to watch the show on Saturday mornings, the younger audiences, towards whom the show's sponsors would try to advertise their products, weren't too interested, which lead to Freakazoid's demise.
Same with Tiny Toon Adventures, which has a huge fanbase of both younger and older fans, and lasted about the same as Animaniacs did.
Little Howard's Big Question is an example of this, possibly due to "Big Howard" Howard Read being a rather excellent "grown up" stand-up comedian.
Jane and the Dragon, which airs on cartoon blocks for elementary-schoolers, has a surprisingly broad demographic appeal. Well-developed characters, storylines that often dodge cliches, and sophisticated dialogue draw in viewers far older than the target audience. A lot of the comments for the Youtube videos say something to the effect of, "I'm [age way older than the target audience], and I love this show!"
The Magic Roundabout - Due to its short length and its position before the Six o'Clock News, there was a huge adult following, whom got into it just because they were waiting for the News to come on. So much so that when The BBC moved it away from the News there was an outcry from the adult audience.
South Park is extremely popular with Yaoi Fangirls, if the amount of Slash fanfiction and fanart featuring its characters is any indication.
Plus, just like with every (or almost every) Disney cartoon, there are many female characters that the male viewers find hot, like Vanessa, Candace, and Stacey for example.
Gravity Falls has received a dedicated following of teens and adults, due to, among other things, Easter eggs and intricate plot details that may go unnoticed to the younger fans.
Also, the style is very reminiscent of many 90's cartoons, which helps bring in fans who watched cartoons in that era.
This is the entire motto AND modus operandi of The Hub. Knowing they couldn't beat the other major kids' networks with just kids watching, it's a veritable fountain of fun Periphery Demographic. Their crown jewel in this regard is My Little Pony: Friendship Is Magic; see the Toys section.
W.I.T.C.H., a comic/cartoon about five girls with magical powers, has no small number of male fans, in part due to its gorgeous art (and not just in a sexual sense, either; it's essentially a series that ascends Disney fairy-tale art Up to Eleven), and in part due to the way its characters, though predominantly female, are also very human. The show took its appeal to the opposite gender even farther; while Blunk was not so liked, Napoleon the talking cat was a big hit, to the point that it's sad his screen antics never ascended in the comics.
Disney's Gargoyles eventually succumbed thanks to this effect. Disney wanted to sell the show to 6-11 boys, but the writing and plots attracted 13-30 year old sci-fi fans. Executive Meddling kicked in and fans (and the Comic Adaptation) ignored the third season.
Animaniacs intentionally invoked this trope; it was marketed to kids, but was also intended for adults, with Getting Crap Past the Radar and old pop culture references in nearly every episode. Yet, it had enough Slapstick and Toilet Humor alongside the Parental Bonus to keep the kiddies entertained, which is probably part of the reason it outlived many similar shows.
Recess, despite being a kid show, attracted many older fans, from teenagers to the parents of the kids who were in the target audience. The show is full of jokes that the target audiencewouldn'tunderstand and has a nostalgic feel for those who remember being in elementary school. This pretty much caused it to be the Sleeper Hit that it is. And it's just getting bigger, now that the original target audience from the late 1990s are teens or adults and still enjoying the show.
Apparently, it seemed as if ABC took note of this. Advertisements for Recess would often air during late night news showings or adult-oriented programing.
It's also popular among adults who work in or teach science, who are relieved to see a science-education show that actually gets things right most of the time.
Adventures from the Book of Virtues, despite being targeted at families with children between the ages of two and nine, is enjoyed by many preteens, teenagers and young adults, particularly those who grew up with it.
Jem and Sailor Moon in their respective eras were both aimed at young girls but also had a male fanbase (which would later be revealed through the internet, because they wouldn't admit it as kids to their friends), the male fanbase having a crush on the lead female character (or another female character) of the respective show.
And of course Jem has an extremely large Camp Gay following for obvious reasons.
The Britt Allcroft era of Thomas the Tank Engine has a very large amount of adult and young adult fans, many of whom grew up with the shows and appreciate the realism and storylines of the earlier episodes. In spite of many Narrating the Obvious moments, the earlier episodes didn't believe that Viewers Are Morons and still treated them with respect.
There are quite a few people that happily collect the train models too.
Many autistic kids also like the show, as mentioned on the main page itself.
Speaking of Britt Allcroft, one of her other shows, The Magic Adventures Of Mumfie, has a lot of teen and adult fans for the same reason. She even mentioned this in her letter on the official website.
Arthur, despite being a show directed toward preteens, has a substantial teen and adult following. This is mainly due to some very smart writing that would go over most kids' heads (such as an entire episode based on Waiting for Godot).
It's a show for preteens and kids, on networks solely for kids and it gets an episode that parodies various Comedy Central(!) shows. Especially notable is the South Park segment.
On the topic of Disney Junior, a lot of babies love Mickey Mouse Clubhouse because of the big heads the characters have and their high-pitched voices. note Babies are naturally attracted to other babies, animals and humans with big heads-this is called baby schema.