I've felt old several times over the past several years, now I don't so much "feel old' as I just accept that I am old.
The two latest things that made me feel old:
I have grey hairs.
A woman at worked asked me about my Black Sabbath tattoo. She didn't know who they were, fair enough, that's to be expected. So I asked her, "do you know who Ozzy Osbourne is?" Expecting her to say yes, because of the TV show The Osbournes. She said no. I'm so old the current crop of young adults don't know who Ozzy is unless they have a real interest in heavy metal music.