Is there nightlife for a foreigner under 21 in the US??

I will be taking a roadtrip with a friend of mine from Costa Rica. We will be in DC, NYC and Boston, and I'm concerned that we won't be able to hang out at night because my friend is only 20. Do bars in those cities typically card heavily?? Any recommendations for non-mainstream bars? Should we be looking for alternate activities, or should I be able to get him into most places?
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