Having to be at least 13 to do math problems is depressing. I may potentially work with a middle school math club next year and I'd only be able to have half of them use your lovely site.
It feels like a "CYA" move since there is obviously nothing age-inappropriate about math problems.
Granted I don't know the legal realities you have to deal with. But is there any hope here? Can you just disable all social features for them?