There are no age limits to sign up (because if there is a young person who is in a situation where they could do with our advice and support, we would much prefer they signed up and talked to us than kept silent - and because third-world countries where polygamy is common also often marry younger than the Western norm). However, we do ask people to indicate an age bracket when they sign up, and some private messaging restrictions are automatically placed on accounts where people state they are young, for their own protection, so any communication they do have here is in public where we can monitor it.
I would suggest that you spend some time here before allowing your daughter to join so you can see the type of content that we have. We can discuss some very adult topics that aren't appropriate for a child. However, you will know your own daughter's maturity level and what is right for her.