You do not have to wait til they are 18. At least not in nc where we are located. Just have to have low income and resources and enough doctor recommendations to prove they need it. My daughter is on it and it took 6 months to be approved. She was in physical, occupational & speech therapy and ASD has to be pretty severe . She is level 3 & nonverbal. I even got a handicap pass for the car because she is in braces due to severe toe walking.

You have to wait until they are 18. (If they do end up qualifying, they will get reimbursed for the months between the one after they turned 18 and the month they finally qualified for SSI.) You will need to take them to a state approved psychologist for testing, who will send SS the results. (You can ask to receive the results, too.) That's the quick explanation. More on the Social Security site. Good luck.