At 11, I'd be more concerned if the kid wasn't getting teased. That would indicate they were a loner, outcast or something was wrong. Everybody that interacts with peers get teased at that age.
That said, you have to find out if this was just dinner table banter or an ongoing thing from a particular person/clique. See if it's still an issue next week, next month.
I just read this afternoon, girls in the US started menstruating, at an average, of 12 and 1/2 in 1939. Bet it's much younger now. Prepare to chat.
