My child's father went to prison right after she was born. I applied for welfare. Welfare sent him a letter for child support which obviously he could not pay. My child is 12 now. If I open the case now, does he owe for all that time or will it start now?

It will start whenever the case is opened. If DCSS opened a welfare case some years ago it will start when the case was opened.

It will start from the time that the case is oppened. If you are just now starting a case then it will start now. If DCSS started a case then it will start whenever the judge signed off on the case however many years ago. However, if they have not been able to collect all this time they will most likely intercept any payments until they are caught up first to pay the government back for what was given to you. (though what was awarded for support won't equal the benefits you received it will catch him up with them)

Mostly it depends on the outcome of the first support order. Was there an amount ordered for support or was it set a 0 because of prison incarceration ?