Minky Momo had a wand that likewise aged her from 8-ish to 18. It also gave her perfect skill with any career she named with the age-up.
Sailor Moon had a wand similar to Minky Momo, used several times in the first 10 or so episodes, and then was promptly forgotten about after the filler arc in the second series. The Senshi themselves are supposed to be older after their transformations, although this is not as drastic as other examples.
Likewise, Minako has a compact that serves the same purpose in Codename: Sailor V. Since Usagi is an Expy of her it's not surprising.
This was the premise of Full Moon o Sagashite. In addition to Mitsuki going from 12 to 16, she also temporarily loses the throat cancer she has as a 12 year old which prevents her from singing loudly. And in order to properly disguise herself during her first transformation she dyes her hair blonde. After that it's just assumed that Takuto and Meroko include the dye when they transform her each time.
In Love Hina Kaolla Su becomes older under the semi-mystical "red moon" and when her "big brother" visits.
This seems to be Evangeline's power in Mahou Sensei Negima!; however it's revealed that her "adult form" is just an illusion. Of course, it's played around with... due to the fact that she's Really 700 Years Old and stuck in her current form because she's a vampire, Evangeline's illusion actually makes her appear younger than she really is... it's just her true form looks even younger than that.
Later in the manga "Age Changing Pills" start appearing, which are stated when introduced to just cast the same illusion Evangeline uses. Evidently, this illusion is fully tactile, extends or reduces actual reach and height, and doesn't cover your clothes (which can end up falling off or getting very tight).
Jungle De Ikou, in which a young student gains the power to become a fertility goddess with the powers of Mother Earth in her breasts.
Yu-Gi-Oh!: When Yugi calls upon his mystical Millennium Puzzle to evoke the spirit of an ancient pharaoh into him, his voice deepens and he grows about a foot in height without raising comment from anyone. Since the cameo of the older Yugi in Yu-Gi-Oh! GX has him look largely the same, this is probably just a visual aid to the audience indicating his maturity. Mocked, as well as everything else, in the first episode of Yu-Gi-Oh: The Abridged Series:
Seto: Wait, did your testicles just drop in the last five seconds or something? What the heck happened to your voice?
In Chrono Crusade, Chrono is usually to be found in the form of a teenage boy. However, in his full demon form, when getting power from Rosette's seal, he is a fully grown and not-unattractive man.
In Tenjho Tenge, Maya is usually portrayed as a really young girl, almost chibified. It seems unlikely that such a child would be sword master; however, when she gets serious she transforms into well-endowed and shapely woman, capable of making members of the male species goggle. This is actually a reversal of the trope: The buxom beauty is Maya's true form. She regularly transforms into the tyke to conserve her strength.
When schoolgirl sportscaster Tsukuda Miina becomes the Space Rabbit Girl Warrior Tsushiro Miina in Getsumen To Heiki Mina, she takes on a notably more... "mature" form.
Hono and Yuki from the Master Of Mosquiton OVA both have an adult form, which seems to be the form in which they can most effectively use their elemental powers. It's used as an excuse for blatant fanservice in the case of Yuki, whose transformation is usually accompanied by boingy sound effects as her relevant curves (shown in extreme closeup) abruptly, and literally, pop out of nowhere.
Ayashi no Ceres has an odd partial example — Aya's friend and fellow ten'nyo Chidori is in her late teens, but physically looks about ten. When she transforms into her ten'nyo form, however, she looks her actual age.
Klein Klan in Macross Frontier does this in reverse. At normal Zentradi size, she's a physically developed adult, but when Miclonised (shrunk to human size) she becomes a physical and mental preteen.
ChaCha in Akazukin Cha Cha can use the power of her brooch (and Riiya's bracelet, and Shine's ring) to turn into a "Magical Princess" who looks to be about 16 years old. She herself looks to be about 7-8.
Nanaka 6/17 doesn't actually have this trope, however it is the explaination given to the main character after she suffers a severe brain injury that leaves her with no memories beyond the age of six.
It actually does have this trope, but only in a minor capacity - Magical Domiko does this when she uses her powers.
In YuYu Hakusho, Koenma normally looks like a baby despite being Really 700 Years Old. When in the human world, in order to be taken seriously, he appears in the form of a handsome young man... with a pacifier in his mouth. He actually has a good reason to always carry the pacifier.
Both Jiyu and Freesia from Jubei-chan have adult alter-forms for serious butt-kicking.
In the Devil May Cry 3 manga, Alice (who resembles the protagonist from Alice in Wonderland) actually goes so far as to trade her innocence to demons for the sake of becoming older and more attractive.
Vivio in Magical Girl Lyrical Nanoha Vivid. If you've seen StrikerSit's the same one she used in the final battle, though with less black in the Barrier Jacket. Like her mom, however, she takes a very responsible approach to life-altering magics and readily promises her to never use her adult form for fun and mischief. She also displays acute awareness of the fact that suddenly growing breasts doesn't actually make her an adult. Other charecters also have a similar Adult Form, like Einhart Stratos, her rivaland maybe something more.
Played straight with Dark Magical Girl Pixy Misa in Pretty Sammy. This is presumably a reflection of Misao wanting to be a daring, attractive grown-up. Meanwhile, the heroine Pretty Sammy is basically Sasami in a silly costume (the cast-offs of her Cloudcuckoolander sponsor). Sasami doesn't want to be someone else, being rather well-adjusted.
Spoofed in a bonus manga chapter of Kodomo no Jikan that came with the 2nd OAV DVD. Poor Kuro remains flat-chested even in adult form, and poor Mimi gets even bigger.
Aram of Meru Puri transforms from a 12 year old kid into a 17 year old teenager by coming in contact with darkness.
There's a borderline example in DNAngel—the main hook of the story is that Daisuke transforms into his alter-ego, Dark, when he experiences the feelings of being in love. Dark looks older and is much taller (to the point that Daisuke gets into the habit of wearing clothing that is too big for him). However, Dark is also presented as his own individual, who has appeared in the men of Daisuke's family for several generations.
When Ahiru transforms into Princess Tutu, she looks noticeably older, and also becomes several times more graceful, with a melodious voice and stunning dance skills. However, it's implied that they are two different characters in the story, since the original The Prince and the Raven didn't have a duck in it, possibly not any kind of Secret Identity at all.
Basically, same person, different personas.
In Pokémon 3: Spell of the Unown, Molly decides she'd like to be older, and they make it so.
In Dorothy of Oz, Mara ages ten years and gains the powers of a witch whenever she uses the boots given to her by Selluriah. Her hair also turns white and her eyes turn gold, for some reason.
This happens in Beyblade of all things with BEGA member Ming-Ming who goes through some kind of magical girls transformation and like Yugi in Yu-Gi-Oh! appears taller and has sharper eyes and slightly different hair. It's a very odd case as she was introduced in the later half of the last season. Notably, the small, cute Ming-Ming is the Idol Singer while the taller, sexy Ming-Ming is the Beyblader.
The trope was formerly named, perhaps ironically, after one of the exceedingly few western examples of it: DC comic hero Captain Marvel. The "awkward situations with adults of the opposite sex" was played straight with Beautia Sivana, but inverted in Justice Society of America in which Captain Marvel reveals his identity to and begins dating a fellow superhero the same age as Billy Batson, and is later forced to break up with her due to misunderstandings from others who didn't know he was actually 16. His twin sister, Mary Marvel, has exactly the same power in some versions. (In the original Fawcett comics she gets the powers but doesn't age, like Captain Marvel Jr. In Jeff Smith's Shazam and the Monster Society of Evil and Mike Kunkell's Billy Batson and the Magic of Shazam, Mary doesn't get older and is also noticably younger than Billy).
Becomes a plot point in Young Justice when he is able to travel between all-kids and all-adults parallel worlds.
Similar thing happened to his Malibu ComicsCaptain Ersatz, Prime from The Ultraverse. He tried to use the older form to get together with a girl he had a crush on, which really creeped her out. Meanwhile, an adult heroine was nursing a crush on Prime.
Ultimate Spider-Man does this... sort of. He's actually 16, but usually tells people he's in his twenties when he's in costume. No one really buys it.
Archie Comics Superhero The Fly (also known as Fly Man) was originally one of these. Young orphan Tommy Troy was given a magic ring that allowed him to transform into an adult with insect-based powers. Unusually for this trope, the age differential didn't last. By the fifth issue Thomas Troy had aged a good ten years, gaining a law degree along the way, and was drained of any lingering trace of originality. (This was concurrent with the departure of the original Joe Simon/Jack Kirby creative team.) When The Fly and other Archie Comics heroes were revived under DC Comics' Impact imprint, this trope was re-established for the character.
Amethyst, Princess of Gemworld is another western comic book example. Because time passed at different speeds between Earth and Gemworld, she was older when she visited the other world. (Plus, of course, she had magical powers! And a flying horse!)
As created by Mick Angelo, Marvelman originally had this, being a Captain Ersatz of Captain Marvel. When Alan Moore wrote the character, he had Micky Moran grown up.
Inverted with Kevin Trueblood/Maximum Man in Welcome to Tranquility, a Captain Ersatz of Captain Marvel who is now an old man, but, when he remembers his magic word, still turns into a thirty year old superhero.
Later, Pikachu also gets the power to temporarily age up using the Thunder Stone...but after the transformation has run out, she temporarily becomes even younger than usual.
One particular That Guy with the GlassesSlash Fic casts Lewis Lovhaug as a high school student who uses a Power Rangers transmorpher to become the superhero "Linkara". While the age difference isn't major, Linkara is stated to be five years older than teenaged Lewis (making him roughly Mr. Lovhaug's actual age).
Flossie Teacakes - the 10-year-old main character finds a magical coat that magically transforms her into the young woman she dreams of being whenever she wears it.
Come On Ossie - Oswald turns into an 18-year-old when he borrows his grandfather's medal.
Live Action TV
In Out of This World, Evie once turned herself into an adult, becoming "Evie's older cousin Yvette," to attract a boy. Attempts to reverse it rendered her seven years old.
In Smallville, an enchanted comic book turned a boy into an adult superhero. Chloe seemed semi-interested in him (with a Flight of Romance, no less) until she found out he was really about ten years old. Turns out, though, that the comic wasn't about a hero, but a villain's Start of Darkness, and the kid was cursed to go Ax-Crazy the moment he felt betrayal.
Justin in Power Rangers Turbo, mostly as a result of the suit-actor being the same age as the rest of the suit-actors, while the character actor was about half the age of the other character actors, with no actual in-story handwave. He even shifted back to twelve-year-old size when he removed his helmet. (We never saw that happen directly, sadly. When the Rangers would be suited but unhelmeted around the base, Justin would be wearing a kid-sized suit, all size-shifting happening offscreen.)
This pertained not only to Power Rangers, but in Super Sentai. Gosei Sentai Dairanger featured a 9-year-old that became the Kiba Ranger with the same circumstances. His living sword spoke for him while he was hiding his identity from the other Dairangers.
The main characters of Big Bad Beetleborgs are all kids who thanks to a Phantasm called Flabber become their favorite heroes. (Note the only the Dairanger example has to get around adult-sized heroes sounding like kids. Even people who know the kids and meet the heroes don't ever say a word about the voice thing.
The titular character of Merlin uses an aging spell now rather regularly to turn into an 80yr old version of himself, known as "Dragoon" most of the time, though Morgana may know him as Emrys due to her vision.
When channeling their dead sister/cousin Mia, Pearl and Maya Fey become vessels for her spirit. All this really means is that the full-sized Mia, bust and all, turns up wearing the younger Fey's clothes. It's bad enough when she turns up in Maya's body, but with Pearl, it gets downright Stripperiffic.
Johnny Garland from Shadow Hearts: From The New World. His actual age is... complicated, but he looks about ten years old most of the time. Towards the end of the game, however, he unleashes his hidden power, and gains the ability to temporarily transform into a rather adult-looking bishonen with a bitchin' double-ended lightsaber, unstoppable superpowers, and no pants. Which goes a long way towards justifying the fact that, in the Best Ending, he ends up living with Shania, a RIDICULOUSLY attractive and voluptuous Native American woman. And yes, he still looks like a kid most of the time. I guess most people choose not to bug them about it, since their combined power is enough to Punch Out Cthulhu.
Will from Illusion of Gaia can transform into Freedan, an adult swordsman. They look nothing like each other, though.
In Transformers: The Movie, this happens when Hot Rod opens the Autobot Matrix of Leadership, which has him grow about a foot and half, sounds much older and he becomes a RV truck in his alternate mode. Though this ends when Rodimus Prime gives the Matrix back to a fully restored Optimus Prime and becomes Hot Rod again. Of course, in the Japanese continuation Rodimus Prime gets the Matrix back and regains the larger form when Optimus sacrifices himself again
Another TF example, this time in Transformers Animated. After discovering that she's a pseudo-Transformer, Sari Sumdac uses her Allspark-enhanced security key on herself to see if she can upgrade herself. She succeeds, converting almost instantly from a girl of approximately 8 years old, to a much taller teenager.
Numbuh Two in Codename: Kids Next Door uses his technology to turn him into a teenager, and ends up in love with Cree. Unfortunately, the transformation is only temporary, and when Cree finds out, she is disgusted that she was on a date with a kid.