In most circumstances, you must pay child support until your child reaches the age of 21. The age of majority is 18 for custody, visitation, and other purposes, but remains 21 for purposes of paying child support.Unless there’s some kind of agreement otherwise, parents have no duty to support an adult child.n our example, the court would multiply $50,000 x .17 = $8,500. That product ($8,500) is the basic child support obligation. You would be responsible for 40% of that figure ($3,400) because your income ($20,000) makes up 40% of the combined parental income ($50,000).