I drank beer in highschool. I can see where the youngster is coming from, but it's a team rule and rules have consequences that can be fairly onerous. Dean however fails to understand that he is putting Jason in a very uncomfortable role and even called him on it; repeatedly. Brothers! Been there and still there, I have 4 of them.
So yeah I get where Dean is coming from and also from where the rest of the housemates are coming from as well. The funny thing is, that when I was in my twenties, they lowered the legal age for beer in most jurisdictions to eighteen and the rationale was that if someone was old enough to get drafted and go to war, they were old enough to drink. Less than a decade later, the drinking age was returned to twenty-one. The number of teenage deaths involving alcohol came down considerably after that. Maturity is not a straight line and doesn't occur predictably by age, but the results speak for themselves.