Um 16ish? yr 10 or 11
But thats cos mum convinced me it would ruin my skin, and since I never had much in the way of pimples etc, and the other kids did, I believed her
Maybe if she is going to insist, and you are going to consider it (though I still think 12 is kinda young, but Im a geek and not "with it") get her a mineral makeup (powder one) or something instead that wont clog up her pores like foundation does.