I think my worst 'feeling old' moment actually occurred when I was in my very early 30s, and demonstrated to me forcibly the significance of a generation gap.
I was teaching a class of 16 and 17-year olds around 1980, and using a textbook that had a lot of pictures that provided prompts for writing. One of them was a clear black and white photo of The Beatles. I asked the class who could tell me the name of that group.
After an uncomfortably lengthy-- and to me, shocking-- silence, I think I blurted out something like, "Oh come on, surely someone here knows the name of this band." I couldn't believe that I had to cajole them to state the obvious.
Finally a young lady put up her hand and hesitatingly replied, "Isn't that Paul McCartney's old group?"
I became a much sadder and somewhat wizened man by the end of that day.