I think I was in my 30s, when Mrs. Bee told me a joke that Niles told on Frasier that had the word "cornichon" as a punchline. I had never heard the word before.

Here's a quick picture and explanation:
http://www.thekitchn.com/whats-the-deal-with-cornichons-117240

Now what's crazy is that Charlie uses the word all the time, as he loves "little pickles" and so Mrs. Bee taught him the word too.

How old were you when you learned the word, "cornichon"?

EDIT TO ADD: Here is the joke!! It was actually by Frasier, not Niles (at 3 minutes, 57 seconds!)