I've been asked if my mum was in by a door canvasser.
I've had a bottle of wine cancelled in a restaurant as the manager didn't think I was 18. I was 31 at the time - when the manager came over to tell me they wouldn't be serving me alcohol I was very polite and asked why ever not, he said "sorry miss, you need to be 18 to purchase alcohol" in a very odd voice; I then told him my date of birth and then pulled my ID out to prove it. I was older than him!
In my last job I was training a new girl, (she was 19) and a manager walked in and looked shocked, and then went to my team leader and demanded to know why two new girls were sitting together with no one training them... she was baffled and replied that "actually, Squirrel has worked here for 7 years" he then shouted across at me "how old are you?" and when I told him he looked really surprised. He didn't say anything else but just walked out again. Everyone cracked up.
Went on a mini cruise quite a few years ago, while walking off the boat there was a school trip, and the students were being waved to one side to form a queue while the other passengers were being directed to the coach for the city visit. I went to walk toward the coach and was told to go back to join the queue of school kids. I laughed and said, "no, I'm booked on this trip and I left school in 1986" and carried on walking.
Think the best one was when I went to my DC's parent's night at school. I sat down with my DS, and the teacher said, to DS, "Did you bring your sister because your mum couldn't make it?" and then DS said "um, this IS my mum". Never seen anyone go as red as that teacher did! This was 5 years ago.
My NDN is positive that I am late twenties. With DC of 18 & 19...?
I do find it a PITA having to justify that I am a bloody adult! It does run in my family though, my Nan looked mid 60's when she was 89.