For what...he is already paying. MOST states do NOT allow a parent to seek CS beyond the date of filing. The ones that do, do so under certain circumstances and then take into consideration the child's age and any "support" given. It is also limited to a year or two at best.
Clark: He can still be chased for c/s for ten years after the child turns 18
If there are no formal orders for CS BEFORE the child turns 18 (19 in some states), then a parent can NOT seek CS AFTER...period.
Sy: 20 years in WA.
That is ONLY if the parent has a formal order AND has arrearages.