The legalities are extremely simple; under 18 means you are the legal responsibility of your parents. Over 18? Do what you want.
Playing while under 18; it depends on who runs the games, as already said. Some places say no because they dont want to bother with the extra insurance.
Some places say yes ONLY if one of your parents is present.
Some places say yes, but Require the signature and authorization of one of your parents.
Read the Newbie Guide.