In a Suit Affecting the Parent-Child Relationship in Texas, the Divorce Court retains continuing exclusive jurisdiction for the custody and support and other SAPCR issues until the child turns eighteen.
A child is deemed to be emancipated or free whenever they turn eighteen. Eighteen is the age at which you are "competent to enter to a contractual agreement."
For example, if there is a five year old boy who is involved in the original divorce in a District Court in Collin County, then when the child is eleven years old the father decides that he wants to file a Petition with the Court and ask the judge to name him primary managing conservator of the child the proper court is the same divorce court in Collin County even if the boy lives elsewhere. There are a number of different reasons for modifications. One, CPS may have intervened and made an allegation that the mother was not fit to be a parent. If it was shown the boy was in danger with the mother, the father could file a suit to terminate parental rights in that same Court in which he was divorced. Even if the father had moved down to Ellis County, the Collin County Divorce Court would retain continuing exclusive jurisdiction.