how old does a child half to be to dicide who he wants to live with ?
The custody decision will be made by a Judge and will be based on the best interests of the child. The wishes of children at least 14 years of age are given more consideration than younger children. Many other factors go into the Judge's decision. You should consult a local attorney familiar with family law in your area for help and specific advice.
18. However, if you are asking in the context of a custody situation, his ore her preference is a significant factor among others in a judges determination as to what is in the best interest of the child.