A parental duty of support is owed until a child reaches 18 or graduates from high school, whichever events occurs later. However, where the child is physically or mentally disabled, unemployable, or otherwise unable to support himself, a parent's duty for support may continue. The burden is on the petitioner to prove that the adult child satisfies one of the aforementioned exceptions.
I don't understand what you mean by a "5 year extension". If you think your child support obligation should be terminated or modified, file a petition with the family court.