Most of the main few start off solidly in teenager range in terms of both age and maturity, but I agree that they develop enough, especially MC, that teenager applies to him less and less as the story goes. Or do you mean that 'teenager' brings connotations of characters being younger?
The one Sarah paizuri art was the first CG I ever got for the game, before she had canon art, and will get replaced eventually. For the rest... idk. this game doesn't really make much money, certainly not enough to kidnap one artist and get perfectly consistent art from one guy forever. I'll keep trying to get more high quality CGs, but there isn't really an easy solution to making sure they're all the same style.
Sarah's non-paizuri CGs represent her better than her sprite does though, and I'll die on that hill, lol.
Glad you're enjoying the game in general though!