I went through a divorce and my ex was awarded primary custody with us having joint legal custody.My daughter who is 16 wants to live with me.Her grades are not the greatest and she says she's happier with me.She doesn't get along with her stepmom that well.What is the law for California for that age to decide?

Please pay more attention to posting guidelines. There is a Read First sticky in the Divorce forum (where this was moved from) that directs questions of a legal nature to this forum.

A child (under 18) cannot decide which parent to live with. This decision can only be made by a court. However, at 16, your daughter's preference will be listened to and considered.

You don't say how long ago the custody was awarded. If it was recent, you may not be able to reopen the case yet. Otherwise you can file a petition for modification of the custody order and request a hearing.