There is no clear cut age when a court is willing to hear from a child. The court will consider many factors including age, maturity, IQ, etc. when deciding to hear from a child in a custody matter. One of the major issues is that must be made clear is no matter what the child says, it is never the child's decision. This is done for many reasons, including the potential long term harm a child may suffer years down the road by harboring a feeling of guilty that he/she made a bad decision or harmed one parent. Children should not have to choose.
I suggest you consult with and hire an attorney to seek modification. Keep in mind as a word of caution that the court may always rule against you.