If child support was never established, once the kids become adults, you can't collect... unfortunately =(
My story about that:
My first husband was extremely abusive - I left him and went to a safe house with my 2 kids (daughter age 2 1/2 and son age 10 months), then went to another state and stayed with my sister. Eventually he found us, telling me how much he'd changed, got "help" and wanted to prove to me that things would be different... Long distance it's very easy to put on a front... I even spoke to his counselor who told me how much he'd changed... we got back together after a while and we came back "home"... he told me he still had the job at the school, he'd rented a 3 bedroom house on a lake, blah blah blah... drove back in the car I had bought, leaving a LOT of our things behind (I had only the clothes on our backs when we got to my sisters, after working and buying wardrobes for the three of us, furniture - some of which was given to me by friends of my sister). When we got back "home".. I found out he'd taken a 2 week pay advance from his job, there was no house on the lake - we ended up in a motel. Moved from place to place till a church helped us get into a place... the emotional abuse started slowly... knowing what would eventually happen, after a bit less than a year, after talking about killing us all by turning on the gas so we could spend eternity together, I left him again - when I did, he held my children captive threatening to kill them and then himself... with intervention, I got my children back safely, and with the help of a mutual friend, got back on my feet, never letting him know where we were. Got a divorce, sole custody of my children and a permanent restraining order (which was pretty much unheard of, but the judge had previously been a criminal judge and had convicted my ex on 5 unlawful use of weapon charges over the years and knew what he was capable of). When he found out where I was living, I moved again to another state. I tried to get an order of Child Support, but they could never find him... when they did find him, he'd move again... so an order was never established. When my son turned 18, two weeks later, he called our house and spoke to him... completely freaking him out - he was a wreck! Told him he'd hired a detective and knew all about us and our lives - knew what schools he went to - everything! How scary! He was in another state and calmed my son down - he was petrified that he would show up at our house. I was remarried and my husband had always said that if he ever showed up on our door he'd kill him. My son never spoke to him again. I called him that night - collect - to say I was livid is an understatement. Because I was on assistance for a short time, he decided that he had supported us with his tax dollars... what a piece of work! My daughter had left home by then and was very angry that he dared to call. I told my daughter to NEVER give him her address - she called him and gave him her cell phone number and told him she needed money over the course of a couple months, he wired a bit over $1000 to her - she said he owed her that - my thought was that whatever she could get out of him - good for her! My son was mad at her for taking the money and not giving it to me, but it didn't bother me... she stopped speaking to him and changed her phone number when she asked for $600 and he contacted his family and asked them to give her the money... when they called her, she told them she didn't want THEIR money... she wanted HIS... so she called him and yelled at him, telling him the same thing and told him she was done with him... never speaking to him again. He has no contact with any of us... not even his family - he screwed them over again - they had disowned him when I left him the first time (he had done some horrible things to them as well) I have remained in touch with them all along - my children should not be deprived of them just because their father was deranged - when they told me he'd contacted them, I just told them to be careful... and they found out the hard way that he hadn't changed... all of them moved when they stopped talking to him again as well... they were afraid of him doing something to them/their homes... he'd burned down his aunts house years ago because they "burned" him <sigh>
Anyway... during this time, I went back to Child Support and asked if they could collect back support - since there was never an ordered established because they couldn't find him, they could not collect on something that was never ordered... convenient that he waited till my son was 18 before contacting... he did his homework apparently.
So... unfortunately - no back child support unless an order was established.