A student, at the start of a close-reading exercise on some 'modern' (ha) literature: "That's really sexist language. And it was probably sexist even in 1995."
Objectively, I already knew that most of my students were born somewhere around 1986. Objectively. But nothing will bring it home to you more speedily than realising that while for you, 1995 meant writing Bon Jovi lyrics in the back of your geography book and quoting Clueless beyond the point of human endurance, for your students, 1995 is another country and they do things differently there.
(Although, we did buy Robson and Jerome singles of our own free will. Maybe it was another world after all.)
I knew I was old when I caught myself saying things like, "Oh my, I can't believe it's April already! It seems like Christmas was just a few weeks ago!" and other such time-related twaddle.
And I've been doing that for years.