How would an eighteen year old go about getting back child support and having it come to them not their coustodien

The eightee old realizes that the money was to raise them and not for them so the only person that can collect the money is the person that raised them. In other words there is no way the custodial parent, custodian, foster parent, who ever spent the money on food and shelter and the other basic needs is the one and only one entitled to the money.

An 18 year old wouldn't. Like Stina said - the child support was never owed to you and if there was no court order it was never owed to your custodial parent either.