Age (In)appropriate

This is a real conversation I had at Best Buy during lunch today:

Me: *places Super Smash Brothers Brawl on the checkout* Good Afternoon.
BB Checkout Guy: Er, um. Shouldn't you be in school by now?
Me: *raises eyebrow* Excuse me? You're kidding right?
BBCG: No, really.
Me: How old, exactly, do you think I am?
BBCG: I don't know, 17? 18?
Me: Are you serious? Try doubling that.
BBCG: I don't know, man. I'm just sayin'...

Okay, the Checkout Guy was CLEARLY younger than I. Was it the fact that I was purchasing a video game in the middle of the day? The lack of the full make-up on this rainy day? I know I look youngish, but I have not been pegged as a teenager since I was in my 20's. Generally, people are about -3 to 5 years from my real age - it's the Chinese genes. Seriously, though, 17 or 18? I'm not sure if I should be complimented or insulted.