The people I know who are over 18 and still copping are a little south of "adult".
Wait, aren't you and I adul-- Ohhhhh, you're limiting said buzz coppage to exclude alcohol.
Don't you think some of your adult acquaintances might be more inclined to cop their buzz from ganja if it were legalized (e.g. Colorado)? See, it's the ol' alcohol vs. weed debate again.