My younger cousin looked at me funny when I noticed he was playing some new version of Pokemon and I told him, with pride, that I remember catching all 151 Pokemon back in my day.
I was going through a movie store with my girlfriend at the time, and saw some movie on VHS. I mentioned that it was a good movie, and she picked it up, stared at it for a moment, and said "It must be really long..." I said "Why do you say that?" Her response? "Look at the size of the box! It has to be on at least four discs!" The kicker? She was only 3 days younger than me. Still, it made me feel mighty damn old.
Another cousin of mine saw my VCR sitting in my room, and asked me what the hell it was. I told him, and he asked me if I ever used it. I told him I did, on occasion. He asked me what kind of movies I still had on tape cassette, and I stopped myself before saying "Well, porn, for one," then realized that I didn't have to stop myself, because this young whippersnapper who had no idea what a VCR was actually 18, and therefore had gone through that magical transformation all people go through at that age where they go from being blissfully ignorant of anything even remotely naughty to being fully aware, and even experienced, in all things sexual.
On that same note, a few months later, I messaged him on Yahoo and he didn't respond. The next day, I had an offline from him explaining that he had been...busy...with his girlfriend at the time and was sorry he'd missed me. Nothing quite like realizing that your cousin, who you can remember being born, is getting more action than you.