Society says some stuff is bad that's not actually that bad.

Children and teenagers should not be able to do some things that adults are allowed to do.

These are not mutually exclusive.

An 18+ rating on Deus Ex is dumb for a mature 16 year old. Cannabis should be legal for adults.

I'm also glad that 12 year olds can't drive cars. Even if I wished I could drive at 12.

In other words, the rules can be set up unfairly AND still have room for just differentiation between children and adults.