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.