"You wouldn't know about that, you're too young" and its opposite, "of course you can do that, you're young."
"Johnny Weissmuller once swam in this swimming pool, but you wouldn't know who he was, you're too young."
(The person telling me this was under 50, which makes her too young to have seen Tarzan in the 1930s, too. And I'm an avid fan of classic movies, especially those from the 20s and 30s, so yes, I know who he was).
"Of course you know how to do XXX on a computer, you've been using them since kindergarten. It's a lot harder if you don't learn them until you're older."
(I attended kindergarten in 1978. I don't recall having computers in the classroom. I learned how to use most programs in college, and yes, I struggled with the mouse at first, too).