It must really suck going to a festival in america and being under 21.
Do some festivals over there forbid you from bringing in alcohol? You can bring all the drink you want into festivals over here as long as you don't bring it in glass bottles. They usually pat down everyone and search the odd bag for drugs or glass bottles but every time i've managed to sneak drugs in
For large festivals they usually don't have the time or patience to search people properly.