With my daughter it started when she was approximately 12 years old. She stopped coming to my place and she never said why. When she was 18 she appeared at my doorstep and asked to stay with me. Then it came to a head with the child support and she moved out again. After that the relationship was cold and then she wrote that letter to the court against me on behalf of her mother.
My EX asked Peter to write a letter as well, but he refused. He kept his distance, because of the court case and he still is keeping his distance.
Paul is in Germany and we keep in touch from time to time. He also wrote a letter on behalf of his mother. When he stayed with me, he tried to make it a condition that I still pay child support to his mother even though he was no longer living with her. I feel that I am being used by kids as long as I was paying. They are all in their 20s. I saw my daughter and Peter on Christmas when we had a family get to geather.
As I am sure you know, at this age, there is no way to force the matter. they must decide on their own they wish to have contact with you. Children very often try to reestablish contact with their alienated parent as they become adults. They also often eventually see through the trash talk of the custodial parent...