How old were you at the time? In the U.S., many localities (and most establishments,) have a policy of verifying age for anyone that "looks under 30". That is, of course, completely subjective, so if you were 28 at the time, they could have been just following policy. When I was 19, with a long beard, I was often mistaken for being in my late 20's/early 30's. I could buy beer with almost no fear of being 'carded', while my 23 year old friends were carded often. My wife, on the other hand, at 35 was often mistaken for being in her early 20s. (My wife is 10 years older than I am, yet people often think I am older than her. When I tell people we are 10 years apart, they usually get it backwards, and comment that I look very young for my age.)
case and point on why some places have a policy that is crystal clear. No ID no Alcohol. It does not matter your age they require you to always show your id.
Personally I think the store made the right move. It is their policy. To many people like the posted I quoted can get away with it.
Heck it is possible for some one in is 19-20 to easily added 10-20+ years to there looks so the safe bet is to Aways card because if a store sells to some one under 21 no matter what the age the person looks like they will get fined no if and or buts about it.