Short answer: After 24 months of entitlement to Social Security disability benefits (or certain Railroad Retirement disability benefits), Medicare starts in the 25th month, and Parts A and B are generally enrolled automatically.
The 24 months count from when disability benefits begin, not from the date the person stopped working. The months do not have to be consecutive under current law. Exceptions: people with ALS get Medicare as soon as disability benefits begin, and people with ESRD qualify through a separate route based on dialysis or transplant.
Until Medicare starts, the person may need COBRA, Texas state continuation, a spouse’s plan, Medicaid or a marketplace plan. Once on Medicare, they can decline Part B, but should weigh the Part B late enrollment penalty unless they have coverage from current employment.
When they turn 65 they get a new Initial Enrollment Period and, if they have Part B, a fresh six-month Medigap open enrollment.