In Machusetts, is it illegal for someone under the age of 21 to be intoxicated?

Yes it is illegal for both of those things. If the cop smells alcohol or weed then he will give you a drug test to make sure that you have it in you and if you test positive then they will take the necessary action to make sure that you get punished. It is a constitutional amendment that the legal drinking age is 21 and if a cop has any reason to believe that you have been drinking (strange behavior, smell of breath) then it is his responsibility to enforce the law on you. It is also illegal to consume weed anywhere in the united states unless you have it under prescription for medical usage. It is a federal law and each state has different punishments for it. If the cop smells weed then he will test you and arrest you if you have any marijuana in your system. Keep in mind that marijuana stays in your system for up to a month so some people who haven't smoked in weeks can still get caught if the cop has reason to believe that you were smoking it.