Majority of stores operate think 25 now, so even if they think you are over 18, but look under 25, you'll be asked for ID.
It's all lumped under one category, as I've been ID'd for bonjela or something which requires you to be 16, and although the girl said I look about 20, because of think 25 she has to use that.
But anyway, almost everyone carries ID, and people MUST be used to being asked for ID by now.. so I don't see any sort of problem with it.