I thought about this (again) the other day, when I remembered looking at my parent's HS yearbook photos. Most of the kids looked like they were in their late 20s, to me. Some could have passed for faculty in their 30s.
I think it's probably more to do with hair style and clothing, than anything else. A lot of people sort of settle into a "look" by the time they hit 18, and just sort of...keep it going indefinitely (with obvious exceptions for kids that went balls-deep into a trend at the time).
I thought about this many years ago, when I was hanging out with a next door neighbor of mine who was in his late 60s. His mannerisms, the way he dressed, the way he carried himself, the way he talked, the sorts of "old man" hobbies he had, etc, etc. It eventually struck me that I wasn't talking to an old man. I was talking to a 25 year old in 1964. He hadn't really changed all
that much - everything else around him did, which made him seem "old".
It was definitely kind of a light-bulb moment for me, that changed the way I perceived people a lot older than me.