Depends on your state laws. In CA I think they can only go back 3 years, but I'm not sure.

Thank you for the first one who reply, I guess I am not thinking straight. I am talking about child support modification not back child support( and you are right the court order was done back in California. How far back can she request for modification(ex child will turn 18 this month.)on the divorce decree she waived her right for any court appeal or changes.

It depends on what you're asking. As for anything after 18, High School or age 19 is the cutoff. http://www.child-support-collections.com/support_laws/california-child-support-law.html Modifications are only retroactive to the filing date, but if this is a new order, than it can be retroactive up to 18 years, under federal law. Some states limit it to less years, provided the mother was not on Welfare, but California is 18 years, regardless of Welfare or not. Plus, they add 10% per year compounded interest on anything you cannot pay off immediately. In 1996, a San Diego based Navy Commander was hit with $90,000 on a 14-year-old that he didn't know existed. The mother was a girlfriend from college. One day she quit school, went home to her parents, and refused to talk to him ever again. When asked why she never told him, it was because when she asked him what he would think of becoming a father, he said that he thought he was too young and not ready to be a father. She didn't tell him that he already was. When he got hit with the order, the first thing was to freeze all his and his wife's assets, leaving them unable to pay their bills. They used 20% of his wife's income in calculating the amount, even though he didn't even know her until after he graduated. They lost their house, both their cars, retirement accounts, kids college account, and anything else that could be taken to pay the order. They attached 55% of his gross income, leaving him with just 30% of his net income, after taxes. He had to move his wife and four boys into a two-bedroom apartment on base. This resulted in another increase in his child support. This was done by taking the off-base retail value of a similar apartment and adding it to the amount used for calculating support. Because of the high rental rates in San Diego, it made a significant difference. With the compounded interest, it will take him decades to pay it off. He will never again own a home, and he won't be able to retire.