No, not talking about infantile behaviour.
There seems to be an orthodoxy in filmmaking that if you want to get someone to look 16, you need to get someone who is 19 or so. That's complete rubbish. It really annoys me when I watch a film, and there are a bunch of 20-something blokes or women dressed in school uniform and acting as schoolkids. I know what teenagers look like; I'm a teacher. But what you get in most films, even some big budget ones, tends to look like adults dressed as teenagers. It's pointless and it's annoying.
If you want an actor who looks 15, get an actor who is 15.