Now it's 18, but it used to be 16, and since the 16 year-old looks so goddamn young and some of the stuff he's in in-game, most people are pretty sus about why he looks 6 than 16 (but it's weirder now that he's 18, since he still looks 6)
I haven't played the game myself, but I know for a fact that people grow up from 16/18 to have at least facial hair or look, you know, older. Heck, my brother has facial stuff and he's 14!
I wouldn't have much problem with the game if the 18 actually looked 18 and not a child that needs potty training in pull-ups. I think that's why so many people are pressed about this game, that the 18 year old looks like he needs milk from his mom.