after 17 years they are now taking child support out of the social security disability check and the child is 21 years old
Your question doesn't give as much information as I'd like. At age 19, a child is typically emancipated so I will assume that there is no ongoing obligation. Even if that is the case, a court still has the power to enforce its orders.
In other words, if there is an arrearage, the court can still punish the parent if they fail to pay on the past-due amount. I strongly recommend you contact an attorney to advise you more fully as you could potentially be looking at jail time. Good luck.