There’s really nothing conclusive. Almost all studies are on rats and mice, not humans – which is typical for science, except that normally they move on to humans eventually (since marijuana is illegal, there’s a limit to the studies you can do). A couple studies say that heavy (once a week to once a day) usage may (always, may, not will) affect the brain, but often times it’s more about how they deal with stress (ie binge eating, like from getting used to “the munchies”), not directly diminishing brain size. And then another study (again, on rats) said THC actually increased the size of certain parts of the brain. So… if there’s a teen, probably don’t have them be a really heavy smoker, if for no other reason than that adolescence is a crucial period for developing other ways of dealing with stress. But the occasional joint at a part/concert? Not really going to do anything.