I think there are many moments that let you realize that you're getting older. When you're a kid, your favorable milestones are 15 1/2 and getting a drivers license, then being legal and able to vote at 18, and then still again 21 and able to drink (depending on what state you live in).
I remember the first time a kid (2 years younger than me), said, "it's not like it was in the olden days, like when you were a kid" I was 18! In my 30's it was my first grey hairs. Now, it's different, I received my welcome card from AARP this year. So, kind and thoughtful of them, wouldn't you say?
So much for one moment. lol, but that's the joy of the question now isn't it!