Screen shot and send a support ticket (not a support community post). Provide a link as well.

Hmmm... What category should I put it in? Inappropriate behavior on Khan Academy?

That's up to your jurisdiction.

Hmmm....How do you know these people are under 13? Are they telling off their age to someone and you just saw them?

It is very risky to tell anyone your age on the internet. If you see a post with someone telling people their age, flag it.

Who are you talking about?

The particular user that initiated the creation of this post said in their bio that they were 8 years old.

I created a support ticket, and it appears that their profile has since been childized.

It could be that they're using their parent's or an older person's account.