Child support that you paid is not deductible, nor are the arrears or interest you are now responsible to pay for the delinquent child support. Child support is not taxable income for the parent who received the support. The fact that the "children" are now adults is irrelevant. Someone had to provide food, shelter and clothing to them when they were younger---since you did not provide it at that time, you are now responsible to compensate the person who was forced to do it without your help then.