Many establishments in Boston and Cape Cod will not serve to out of state visitors under 25. Get a Liquor ID, a state ID card available to non-residents from the Registry of Motor Vehicles if you'll be in the area for awhile, costs $25 for five years of validity. Note that under Massachusetts law, the only acceptable proofs of age for purchase of alcoholic beverages are the Massachusetts driver's license and the RMV Liquor ID card. Out-of-state driver's licenses, and even passports, are not acceptable.".
However, I heard that the Massachusetts Liquor ID is not always accepted as a proof of age (entrance to nightclubs/bars or buying alcohol at liquor stores). How likely is it?