I can't figure out a specific trope to associate this with (it may apply to several), but there's actual research suggesting age isn't an easy thing to judge from looks alone.
This probably has relevance to things like 'artistic age', 'older than they look', 'younger than they look' and so on as long as the examples aren't too extreme.
Basically, the issue is that, if you say, had amnesia, and no identifying documents or anyone who knew you, how could you work out how old you actually were?
It turns out that's not so easy to do. Measurable physical traits are limited such that the age group of 16-34 or so actually has no physical traits that can reliably tell you their age.
(Wisdom teeth being one of the few ones, but the variation in when people get them is such that about all you can tell is if someone has all 4 they are over 18. But NOT having all 4 is ambiguous for a lot of reasons.)
Since that's a 14 year range, and it overlaps with a lot of 'legal' definitions, (Age of consent being 16-18 typically, and other legal ages tending to be 18-21. And definitions of 'child' frequently being 'under 18') there's a lot of headaches involved in working out anything to do with people in this age range.
Common laws against depictions of child sex being an obvious problem in that they often define 'any depiction that appears to be of a person under legal age', with that age frequently being 18, yet there being no real objective way to tell if someone is under this limit unless they are <i>well</i> under it.
Animation obviously makes this problem much more serious, since abstracting any kind of small-scale details makes everyone look universally younger, but that's already mentioned in several tropes.
Is there a trope for shrinking with age? Basically As a person get older (usually in anime) they get smaller. And it goes beyond "People were shorter back then" They show young versions of them who stand roughly 5' and up, and then they hit 60 and bam they're 4' nothing. My most recent viewing example is in FMA:B with Pinako Rockbell (Winry's grandmother) in one episode we see a younger Pinako drinking with Hohenhiem (standing around 6") with the lady in question standing up to his shoulder. Then in the modern time of the series, she's like 4" This is just one example.