How about not treating me like a criminal before you've seen my ID?
Why? Because you don't know my age.
You've got some vague idea of how old I might be (e.g. I'm probably not 85), but get your head around the idea that just because someone appears to be under 21 doesn't mean they must be under 21. That little ticked off smirk and "You're underage and I know it so I'm about to bust you" tone of voice really isn't necessary.
That sheepish "Huh huh, der I guess you are of legal age" routine you gave me after seeing I turned 21 half a decade ago was just darling but I'm tiring of it.
Don't be stupid.