I think the first thing that really had this effect upon me was my little brother leaving behind the teenage years, he will within the next year be leaving behind his 20s (sorry for announcing this to the world Mark).
One of the biggest impacts to help me feel old though was also several years ago running a youth group who were putting the latest Now album on the CD player (somewhere around Now 35) and they asked was there a now 1? to which I replied yes, it was just called Now thats what I call music I have it (its a great album) Then came the question whats on it? it was at this point that I realised most of the young people I was talking to at the time were not even born when it came out and probably would not understand the concept of Vinyl either!
The latest was this:-
Being a Dad is great and I dont fell old because of being a Dad, but a Dad whos son wears a school uniform, Im not ready for that! (sorry to all those for whom this happened a long time ago and I have now made you feel even older)