> what about someone who is 18 and just doesn't have the traditional adult facial features?

This can be challenging even with humans. My ex got carded when buying alcohol well into her mid thirties, and staff at the schools she taught at mistook her for a student all the time.

I grew a beard when I was younger because I was tired of being mistaken for a highschooler its quite annoying to have people assume you are 15 when your 20. still regularly carded in my 30s