You should consider this interaction as an opportunity to also revisit your ageism. I'm all for encouraging young people, and OP seems a really bright 13 years old person, for sure. But try to keep condescension out of the way. As anybody with children can tell you, we all learn from them all the time. It's not something surprising.

Maybe don't throw -isms around so lightly, eh?

I find hard to interpret "A 40 yo learned something from a 13 yo" in a non ageist way. Seems that people disagree, but nobody has provided a counterpoint.

Actually, the comment in question would have been perfect encouragement in my view just by removing that phrase.

I've only ever heard that term to mean bias against people considered too old for a given activity.

That's the most commonly discussed form. There are others as in https://www.medicalnewstoday.com/articles/ageism

Just felt like a bit of an overreaction to me.