Frequently child support orders will run to age 18 (there are some states that I think it can run to 21), but you should always check your divorce decree first. Some states do allow for continued child support to continue while the child is attending college but there are usually a number of requirements. These include having a certain number of credits per semester (possibly full time status), the child may have to reside at home, etc. Support usually only covers support and not the cost of tuition.
A court can also declare a child emancipated. You probably need to go to court to get this done.