When I was in school, there was a microbrewery/restaurant a short walk from campus that *never* carded. They had a huge business
But, most bars in Boston are 21+ (some are 23+) and they card *everyone*.
I did once bluff my way into a 19+ bar when I was 18, by showing my college ID and saying I'd left my license at home... (it was 19+ basically to keep high school kids out)