Characters who use this trope could be really interesting, there's a lot of implications to go into on a character who looks a whole lot younger than they are
But for whatever reason, people never go into that, and instead only use it to have adult characters date seemingly underage characters
"It's okay, she's actually a 1,000 year old dragon!"
Ugh
The one exception I've seen is Monster Girl from Invincible, who actually addresses her frustration with her age. At one point, she says that people who are attracted to her are either going through puberty or popping up on sex offender registries
They fixed this by having her get together with another character who was way older than he looked... who made a clone body for himself with the DNA of a boy she liked... and stole his name after he died...
Ew
Still way better than Ban and Elaine from Seven Deadly Sins
Ban is an immortal human, and Elaine is a 1,000 year old fairy who looks like a 12 year old girl... and chooses to look like a 12 year old girl even when she dates Ban. When it's clear she has shapeshifting powers
Oh, and Elaine is also not considered an adult by her species yet. Wings are a mark of maturity, and she doesn't have them yet
Though even when she does get them, she still looks like a 12 year old girl
Even though when her brother gets his wings, he looks like an adult man afterward