At what age can a kid have a say where they want to live?

A couple years ago my 2 oldest boys wanted to try to live with there mom because she was never around. Then me and her started talking hanging out because she said over and over she changed so after many months later she was doing really good so we went and had papers drawn up so she could have custody so that's what my boys wanted and i always told them whatever makes them happy so we had it done and it went good for a while. Then she got into some trouble and i was working out of town and didn't know about it so as of course my sneaky mother found out and went to court and got guardianship of them because she was in jail and i was out of town and didn't get to go to the court date so my oldest son who just turned 11 tells her all the time that he hates her and wants to live with me but she fights him about it and tells him that she will never let that happen so now i need to do whatever I can to get him home to me because he isn't happy and it's so wrong of my own mother to do what she's doing. Someone please help me with my very messed up situation it will be greatly appreciated, thank you.