I'd bet there's a 'sweet spot' for age where the average person watching a movie can mentally overlook the adult in a teen role, while children and elderly can't be portrayed by a different age without it being a deliberate effect choice or farcical (though when I was learning makeup effects, I saw a ~25 year old turned convincingly into a 60+ person). Maybe it has to do with ease of an adult actor compared to a teen, or maybe it's because there are just more of the 'young' adult actors in the pool than readily accessible teenage actors. Maybe the hiring team wants to ensure they have someone who can act without being taught during the production, and the slightly older actors have more proven track records?