The problem I see with 'card everyone' is that it lends itself to cashiers/servers not paying close attention to the ID, as they are so frequently IDing people. They should focus their time/effort on people who could actually be underage.

When I was 17 I worked at a drug store that carded everybody for cigarettes. I ignored the rule and applied common sense, but I would routinely see 17 old girls ask for the ID of people in their 80s. I would ask them, 'do you really think there is any chance this elderly lady is our age?' Total waste of time and energry of the cahsiers and the managers who chastised us for not IDing senior citizens.