i signed my rights away 22 years ago. My son is 24 now. How can I still be sued for back support?

It depends on what it really means or what really happened when you say  you "signed [your] rights away."  If you voluntarily relinquished your parental rights and have a court order confirming such relinquishment, you cannot be held liable for child support.  You should supply that order to child support enforcement.  If you and the child's mother simply agreed amongst yourselves that you would not have anything to do with the child, that is different.  In that case, you are and have still been liable for child support.  If you have a written agreement between you and the child's mother, that may help your cause depending on what is included in the agreement.  Considering that you are facing $160,000 in payments, I would highly suggest you consult an attorney to discuss the specifics of your case and determine your options.