Medicare A is automatic at 24 months after you start receiving your SSDI payments, 29 months total and it is free. You pay for Medicare B, about $96 per month now, but can deny it. You must have proof of creditable drug coverage if you pass on Medicare D, equivalent to Medicare D.

There are no age restrictions on SSDI for Medicare.

It can be very confusing, but once you understand things it is fairly easy.