I remember a quarrel at Giant Food some time ago. A scruffy looking man who certainly seemed well over 21 was complaining because they wouldn't sell him beer or wine (I forget what, exactly) withut ID. A manager was backing up the cashier, along the lines of, "I know you look old enough, but since she asked for ID, and you don't have any, we're not allowed to sell you the alcohol."

On the other hand, they don't always bother asking, if it's someone who clearly looks old enough.