At the Superhero School Whateley Academy in the Whateley Universe, Generator (Jade Sinclair) and Warlocke are both fourteen, but look only ten or eleven. Then there's Headmistress Elizabeth Carson, who looks like a hot movie starlet in her early thirties, but is actually over seventy. And there's at least one teacher there who's Really 700 Years Old: Circe. The Circe.
18-year old Survival of the Fittest character Kimmy Redmond is described as looking about thirteen, which is simply down to physical immaturity rather than anything fantastical. There are also Laeil Burbank, who looks about 16, and Alice Jones, who is commonly mistaken for an elementary school student. A recent addition is V4's Celeste Beaumont, who looks like an under-developed freshman.
This trope is in full force in Deucalion Chronicles - to the point where it's easier to name characters that actually look their age. Almost everyone stops aging after a certain point, thanks to either natural magic or the alchemical solution Ambrosia. Some Outsiders barely age past puberty. The only exceptions are certain religious groups that refuse Ambrosia and non-CU worlds where it is unavailable.
Enrei Islio, 21 in Chaos Fighters II-Chemical Siege is called an 8-year old man due to this trope.
Doug takes it to bizarre levels with Donnie DuPre from Demo Reel. The character is 42, but as he's still wearing tight clothes, hiding his hairline with a fedora and only has a little more stubble than usual, he looks late twenties at most.
Played for laugh with Barbie: Life in the Dreamhouse. In the first episode Barbie says she's celebrating her and Ken's forty-third anniversary, despite the fact she looks no older than 24.
Todd in the Shadows would often mock bad lyrics that sound like they were written by a teenager, but would excuse them believing the person that wrote & sung the song was a teen. Only to find out the person in question was in their mid-to-late 20's (such is the case of Kimberly Perry and Carly Rae Jepson)
Customer: "[...] I even quit drinking because itís too much of a hassle to buy beer!"
In a similar vein, this story has a customer accusing an employee of being a teenager playing hooky. Both the employee and her manager confirm that the employee is 21 years old and a college student but the customer refuses to believe them to the point that she actually calls the police to try and arrest the employee for truancy. Fortunately the police officer realizes this is a bogus charge, and, after verifying the employee's age has the customer banned from their store.
Spinoff site Not Always Working has several instances of guys being mistaken for pedophiles because their twenty-plus-old girlfriends are mistaken for preteens or young teenagers.
The Chakona Space setting features the contributor series, Tales of the Folly. While Neal Foster falls into the Really 700 Years Old category, many of his friends and colonists and all 12 of his Rakshani Marines fall neatly into this trope.
If anything in Doki's Chronicles is to go by then Toki and Doki are really somewhere in their eighties or nineties, as back in the time when these two were born cerebral palsy and selective-mutism were not explainable and were beyond their birth-parents comprehension, except for by a horrible disease (Doki's case) and being a 'good child', as seen and not heard (Toki's case). To further point this out, they should not be able to have babies but did. However, there is a reason as to why the are older than they look and that is because they were frozen when they were 12 and a half years old, drastically slowing down their aging process and causing them to age as if nothing changed.
Also, by the same one, we have Killerbunnies XI and XII: Kamu and Kanran who has a condition that makes them look like babies but they don't seem to let them stop them in being damn near malicious and that both are old enough to smoke but their ages are never stated.
In Jeanne's case, this is unclear, as we don't know if she is a child or if she just looks like one.
We also have Andibea, who's stated to be twenty-two.
Ultra Fast Pony uses this as a joke. Spike (who is stated to be a "baby dragon" in canon) turns out to be 67 years old.
Chris Bores, who is known as the infamous Irate Gamer, was born in 1979, making him older than many of his more well-known colleagues and detractors (and is much older than the majority of his audience by nearly 20 years).
Yang's mother from RWBY uses the same model as her and thus looks a good twenty years younger than she actually is.
Dan Avidan from Ninja Sex Party and Game Grumps is in his late 30's, and his partner Ninja Brian is even older (he's even married with a young daughter). In Brian's case you'd know, but looking at Dan (a peppy human Q-tip who's constantly happy) you'd never guess. This of course is before he starts talking about Rush and calling game levels "boards", which is when he shows his age.