they don't HAVE to put it in...I know some cashiers at a nationwide supermarket and when I buy beer there(yes, I am 21)...they just hit another button and it shows up on the receipt as "cashier verified age" instead of my DOB...I think they just have to cover them selves, as rocksurfer said.
Well that's what it does at every other supermarket I go to as well. But when she tried doing it at this store, wish I could remember what it was. Might have been a Super Walmart. It wouldn't allow her to continue the transaction, and gave her some weird error message until she would put in a date. I should have made something up.