Pot culture is a beautiful thing, and not something you "grow out of".
If you wore rasta gear and bragged about blazing all day when your were younger just because you thought it was cool, then its something you grow out of. But if you wear the Bob Marley shirt just because you love Bob, and tell people you smoke when they ask just because you aren't ashamed of it, then you will hopefully never "grow out of it".
Bottomline is you should be yourself.. To some people that means being dressed up in "prep" clothing, and to others it may mean being decked out in "hippie" clothes. Pre judging someone for how they look is stupid, regardless of the "style".
There are always going to be certain people who dress or act a certain way in an attempt to be "cool" or fit in, but the idea that wearing the red/yellow/green to school somehow makes you immature is a foolish notion IMO.