It depends on where you live.  If you are an under-65 Medicare beneficiary due to a disability or ESRD, your rights to join a Medicare Supplement will depend on your state's Medigap law.

The current information from the Centers for Medicare and Medicaid Services (CMS) shows 33 states allowing under-65 Medicare beneficiaries the right to join at least one type of Medigap policy – with insurance companies in other states (such as Illinois) voluntarily allowing Medicare beneficiaries under 65 to join their Medicare Supplement plans.

(See - https://www.medicare.gov/health-drug-plans/medigap/ready-to-buy/when)

However, no matter where you live, once you turn 65, you can participate in the Medigap Open Enrollment Period and have the guaranteed-issue right to join any Medigap policy available within your state.

For more information, you can contact a local insurance agent or State Health Insurance Assistance Program (SHIP) and ask about your rights to join a Medicare Supplement. You can click here for an overview of the benefits offered by different Medigap policies.

Here is current list of states allowing a Medicare beneficiary who is under 65 to join at least one type of Medicare Supplement plan (as of July 2023):