How old does a child have to be to chose to live with a parent?

First the child never gets to choose, he gets to testify in court as to who he wants to live with, and both parents can still submit other evidence as to what they believe is best. Often all things else being equal, the judge will allow what the child is requesting,

So basically there has to be a motion to change custody done in court, and allow the child to testify.

The age that a judge will listen varies by state and even by court, some will listen to all kids, but put more importance at 11 or 12. Some slightly older

I myself have been through the exact situation... so you know Fr_Chuck... I was 8 when I asked my father to fight for me... I had my own lawyer that questioned me on what I wanted to do. My father gave evidence on my mother being an unfit parent and visa versa with my mother. They then observed both me and my mother in a room full of toys just to see how we were with each other, and visa versa with my dad, in the end they gave my father full custody...
I was in court when I was 8 telling the judge my mum and dad and lawyers exactly what I wanted and it went all in my favor