I am a law school graduate currently awaiting Bar results. What I offer to you is mere information, not to be construed as forming an attorney-client relationship.
Hmm, sounds like you and Yobocker have something in common (you should read his posting too).
18 is typically the age of adulthood, which means technically speaking, court ordered child support should stop at that time. However, does your child have college plans or business plans? Maybe you might think of paying or helping to pay for your child's education. Don't think of child support as a chore, it is supporting a human being, a human being you created. Just because legal enforcement stops at a certain age, it doesn't mean moral obligations stop.
hope this helps. and p.s., ask a lawyer in your state whether their family code has special exceptions (i.e. if your legal obligation does not terminate at the child's turning 18)