Too Tired to be under 21? What?

Apparently, I'm an "old dog". Or at least I look like one...let me explain...

I was at the grocery store the other day and I was purchasing some adult beverages (not for me, of course, it was a "gift") and I asked the cashier if she needed to see my identification. (I remember a time when this annoyed me but I have now reached "that age" where it's fun to be asked for your ID).

Well, she said that she didn't and then said, "You look too tired to be under 21."

Ummm....what? What does that mean? Should I be offended? I wasn't at the time. I just laughed and replied, "amen sister". But now I'm not so sure.... :)