Up to what age of child, do I have to pay child support

18, longer if the child is living at home and attends college if your state laws require.

Or to 21 in some states.. you need to check.

It will all depend on the exact terms of your child support agreement,

If you are in arrears you will have to pay till caught up

That's true, even if the child comes of age you are still responsible to pay all back child support.