Absent marriage or other acts which would emancipate the child, child support orders continue until the child reaches age 18. If the child is in high school at age 18, support continues until high school graduation. If the child is disabled, it may be possible to continue child support for an indefinite period. Texas law makes no provision for support during college, or the payment of college expenses. However, this can be done by a contract between the parties if an agreement can be reached on this issue.
All orders concerning the children are modifiable in the future.
Either parent can petition the Court to change conservatorship, periods of possession, or child support at any time until the child is emancipated. The burden of proof are different in each case, and sometimes are very onerous. You should not enter into an agreement based on the assumption that it can always be modified later. You also should not enter into an agreement that you will pay no or a minimal amount of child support based on the assumption that a request for full child support will not be made later. If you rely on the agreement of your former spouse that you can pay a lesser amount of child support, you are likely to find yourself in contempt of court, as you have violated the Court's Order.