Can you collect back child support if the child is over 18?

Was there ever an order for payment of support?  If so, then all child support ever due for this child remains due -- plus penalties and interest.  The statute of limitations for child support was abolished in 1981.  See http://willicklawgroup.com/interest-penalties/.  If no order was ever entered, however, it is probably too late to seek one now, because the "retroactive" statute, NRS 125B.030, only allows reaching back 4 years. If you have tried and failed to actually collect from the obligor -- and if he actually has anything -- you should consult with an attorney, in this office or some other one, who is well versed in collection of arrearages.