Even when you buy online, they don't really require much proof... Just click on the drop down box and fill in your birthdate.. Of course you use a CC.
This will be really hard to validate if not impossible. Especially if that black out all the info. Even if they don't black out info, what is to stop them from copying someone else's license who is of age?

I think all you can do is have them fill out a form that says they are and then trust or not.. It's up to whomever is dealing with them.

My .02