Recent Dialogue Exchange (or, "How I know I'm Getting Older")

Here's my recent exchange with a clerk when picking up a six-pack of beer:

Clerk: "I need to see your I.D."

Me (playfully): "I love when people ask to see my I.D. Thanks!"

Clerk: "I didn't say you look under 21. It's just the law to ask."

Me: "Oh."