My ex husband and I have joint custody. He makes like $16 + per hr at least 40 hrs week and is in CA. I am in OR on ssi &ssd making 695 mth due to 5 mental breakdowns he caused me to have. He is saying I need to pay child support. I can't afford it. Also how old does a child have to be to decide who to live with?

First, a child CANNOT decide. Only a court can determine who has custody. When a child reaches majority (18) then they are free to live where they want.

Second, There are two types of Custody, Joint and Physical. You may have Joint Legal custody, but living in two different states, I doubt if you would have joint physical custody. One of you must be the primary custodian. And generally the primary custodian is entitled to support. Support is generally based on income. But how much support you might have to pay will depend on which state has jurisdiction.

If you have a divorce decree providing for child support, he is going to have to prove changed circumstances in order to get the child support changed. What child support is ordered now?