Being thought of as under 21 (or worse, since the legal age to drink or smoke here is 19) when you're actually 41 is both flattering and annoying.
I don't get carded all that often these days, but - and those of you who remember this story from a few months ago can skip the rest of this post - when I was in Atlanta at Labour Day for Dragon*Con, I went into the Hyatt bar and ordered a Guinness. The bartender asked me for ID, so I handed over my Ontario health card (which you're not really supposed to use as proof of age, but I don't have a driver's license and my passport was in my room, at another hotel). She looked at it, started to hand it back, then her eyes got really big (we're almost talking "Roger Rabbit" here - imagine a bored Gowron) and she pulled it back again to look a second time. She then apologized, handed it back and poured my beer.
It generally happens a couple of times per year.