This assumes that you're talking about an arrearage, not retroactive support: 1) pay the past-due amount; 2) file a request for account review with your State's child support agency to establish the past-due amount - file an appeal if you think the results of that review are incorrect, or; 3) get the other parent to "compromise" the arrearage (this will require the court's blessing).
from child support enforcement
18
Child support arrears do not expire until they are paid.
An attorney is not needed. Notify child support enforcement and make an official request to modify. You can also do it Pro Se using forms from the Clerk of the Court. see link
No, it is still owed with interest. The back child support was supposed to be paid while the child was growing up. Check your state laws.
No
No. Back child support is money you should have been paying when you didn't. You owe it until it is all paid up.
Yes, it can.
They might.
Yes, nationwide.
NOT if you owe back support. It doesn't matter how old the child is or whether or not she has children IF you owe back support you MUST pay it
You stop paying for child support when your child turns 18.