What Happens to Back Child Support Once My Child Turns 18?

My son is only 10 years old but I'm wondering what happens to the back child support (arrears) once he turns 18. I'm currently owed $26,000+ in arrears and I probably won't ever see a dime of it but does it "go away" once my child becomes an adult? Will I always be owed this money? (until it's paid of course)