Can I ask for back support for a child when my lawyer said he would file, did not and now the child is 18?

A little confused by your question. At the time your child was in the custody of the state were you divorced from your ex? Had there been a child support order in place? What happened to it while the child was taken from you? Why would you need a Modification of Support if you already had a support order in your favor? Was the child in your exes custody and, therefore, there was no support order in place for you to receive child support? With all the problems it is hard to answer your issues. To begin with, if there was no support order in place for you ex to pay you it is now too late to ask for child support in most states. If you were to receive support and your attorney did not do his job you might have a malpractice lawsuit against the attorney for the lost income. I would suggest you talk to a personal injury/family law lawyer to see exactly what may or may not be done in your case.