At what age can a child decide at which relative's house he wants to live at, provided the home at which he wants to live is socially acceptable?

That age is 18, at any age before that only the judge can decide what house the child can live at. Now the judge can allow the child to say what they want, but the judge does not have to do what the child wants.

The parent without custody will have to file a petition for custoday of the child. Then at the court hearing the child can tell the judge what they want, And of course both parents will testify as to what they want and why.