Medicare is a U.S. federal health insurance program mainly for people 65 or older, or for younger people with certain disabilities or specific medical conditions.
You may become eligible for Medicare based on one of the following:

• You are 65 or older
• You are under 65 with a qualifying disability. Medicare eligibility starts after 24 months of receiving Social Security Disability Insurance (SSDI).
• You have a diagnosis at any age of end-stage renal disease (ESRD) or amyotrophic lateral sclerosis (ALS,also called Lou Gehrig’s disease).

All Medicare applicants need to be a U.S. citizen or a permanent legal resident who have resided in the United States continuously for at least five years.

If you receive Social Security or Railroad Retirement Board (RRB) benefits at least four months before your 65th birthday, you’ll be enrolled automatically in Medicare Part A and Part B. Rules are different for residents of Puerto Rico.

You’ll receive your Medicare card in the mail and can start using it the month you turn 65. If your birthday falls on the first day of a month, your coverage will start a month earlier. If you hold off enrolling in Part B, send back the Medicare card and enroll later.

If you’re not getting Social Security or RRB benefits at 65, you can sign up on the Social Security Administration website during your initial enrollment period or a special enrollment period if you delayed enrolling while you had employer coverage. If you miss those deadlines, call Social Security at 800-772-1213, or visit a local office to sign up during a general enrollment period.

You may not have to enroll in Medicare at age 65 if you or your spouse is still working and has comprehensive health insurance through that employer. The rules depend on the size of the employer:

If the company has 20 or more employees, the employer coverage is primary and Medicare is secondary. The employer can’t require you to sign up for Medicare at 65.

You’ll qualify for an SEP to sign up for Medicare without penalty while you have the employer coverage and for up to eight months after that job-based coverage ends.

Even though you aren’t required to enroll in Medicare at 65, many people sign up for Part A during their IEP because you typically don’t pay premiums for Part A. You can delay signing up for both Part A and Part B if you want to make tax-advantaged contributions to a health savings account (HSA).

If the employer has fewer than 20 workers, it may require that you enroll in Medicare Parts A and B. Your work insurance often becomes secondary and fills gaps in Medicare coverage.

You’ll also have to sign up for Medicare at 65 if you have:

• The Affordable Care Act’s (ACA) health insurance marketplace. When you turn 65 with ACA, you must transition from an ACA plan to Medicare.
• Consolidated Omnibus Budget Reconciliation Act (COBRA) coverage. If you lost your job and signed up for COBRA to extend employer insurance, this doesn’t protect you against late penalties for not signing up for Medicare.
• Retiree health insurance. You typically need to enroll in Medicare at 65 for these programs to help fill in the gaps. (Special rules apply for federal retirees.)
• Tricare military health insurance. You must enroll in Medicare Parts A and B when you’re eligible. Then Tricare for Life fills in the gaps for some services Medicare doesn’t cover.

Enrolling in Medicare Parts A and B is the first step, but you’ll need to make other decisions.

Original Medicare or Medicare Advantage? Do you want coverage from original Medicare, the federal program that allows you to use any doctor who accepts Medicare, or a private Medicare Advantage (Part C) plan, which has limited provider networks but usually includes prescription drug coverage? Most Advantage plans offer some dental, hearing and vision benefits.

Original Medicare decisions. If you choose original Medicare, you may need to get a separate Part D prescription policy unless you have comparable drug coverage from an employer, former employer, Tricare or other source.

You may want a Medicare supplement policy, also known as Medigap, to help pay Medicare’s deductibles and copayments. Private insurers sell both types of policies.

Look at the steps to choose and enroll in a Part D prescription drug plan and Medigap policy before your 65th birthday.

Medicare Advantage decisions. If you choose this all-in-one private coverage, you won’t need extra policies. You’ll have several plans to choose from in your area with different coverage, costs and provider networks.

Here, too, learn the steps to choose and enroll in a Medicare Advantage plan ahead of time.

You can sign up for a Part D or Medicare Advantage plan only at certain times after you enroll in Medicare. You can switch plans every year during open enrollment Oct. 15 to Dec. 7 for new coverage starting Jan. 1.

You can buy a Medigap policy at any time, but insurers in most states can reject you or charge more because of preexisting conditions if you buy a policy more than six months after enrolling in Medicare Part B.