Who Is Eligible for Medicare and When Coverage Starts

Medicare covers people 65 and older, some younger people with disabilities, and anyone with end-stage renal disease or ALS, regardless of age

Medicare is a federal health insurance program run by the Centers for Medicare & Medicaid Services (CMS). You do not automatically get it when you turn 65 — you have to enroll during a specific window. The program has four parts: Part A (hospital insurance), Part B (medical insurance), Part D (prescription drug coverage), and Part C (Medicare Advantage, an alternative to A and B). Who can enroll, when they can enroll, and what they pay depends on which part and which category they fall into.

The largest group is people 65 and older who have worked long enough to have earned Social Security credits. But Medicare also covers younger people in three other situations: those with a disability recognized by Social Security, those on dialysis or with a transplant for kidney failure, and those with amyotrophic lateral sclerosis (ALS). Each group has different rules about when coverage starts and what they must do to enroll.

Key Takeaways

You must enroll in Medicare during your Initial Enrollment Period, which runs three months before, the month of, and three months after the month you turn 65 — missing this window can lock you into higher premiums for life.
If you are already receiving Social Security benefits when you turn 65, Medicare Part A and Part B enroll you automatically, but you still need to choose Part D coverage for prescriptions.
People under 65 with a Social Security disability award can enroll in Medicare after 24 months on disability, and those with end-stage renal disease or ALS can enroll when ready upon diagnosis.
Working past 65 does not delay Medicare enrollment if you have earned enough credits — you can enroll and keep your job, though your Part B premium may be higher if you delay.

People 65 and older with work history

You are may be able to access for Medicare Part A at 65 if you have earned at least 40 Social Security credits through work. One credit is earned for each quarter you paid Social Security taxes, so 40 credits typically means 10 years of work. You do not have to be retired to enroll — you can still be working. Part A covers hospital stays, skilled nursing facility care, hospice, and some home health services.

Part B (medical insurance covering doctor visits, outpatient care, and preventive services) is available to anyone may be able to access for Part A, but it is optional. You pay a monthly premium for Part B, and the amount depends on your income from two years prior. If you delay Part B past your Initial Enrollment Period and you are still working, you may owe a higher premium permanently. If you are not working or your employer has fewer than 20 employees, you should enroll in Part B when you turn 65 to avoid this penalty.

Your Initial Enrollment Period is three months before the month you turn 65, the month itself, and three months after. If you miss this window and do not have a valid reason (called a may have access to life event), you cannot enroll in Part B until the next General Enrollment Period, which runs January 1 to March 31 each year, with coverage starting July 1. Missing enrollment can cost you thousands in delayed coverage and permanent premium increases.

People under 65 with a disability

If you receive Social Security Disability Insurance (SSDI) or Supplemental Security Income (SSI) based on a disability, you become may be able to access for Medicare after you have been on disability for 24 months. This 24-month waiting period starts the month your disability benefit begins, not the month you applied. You do not have to do anything — Social Security will enroll you automatically in Part A and Part B once the 24 months are up.

You still need to choose Part D (prescription drug coverage) during your enrollment period, which is the same three-month window as for people turning 65. If you do not choose a Part D plan during this period, you may owe a penalty if you enroll later. Part D plans vary widely in cost and which drugs they cover, so comparing plans before your enrollment period ends is important.

People with end-stage renal disease or ALS

If you are diagnosed with end-stage renal disease (ESRD) — kidney failure requiring dialysis or a transplant — you become may be able to access for Medicare when ready, regardless of age. The same is true if you are diagnosed with amyotrophic lateral sclerosis (ALS, also called Lou Gehrig's disease). You do not have to wait 24 months or meet any work history requirement.

You must enroll yourself; Social Security does not do it automatically for these conditions. Contact Social Security or visit your local Social Security office with proof of your diagnosis (a letter from your doctor or a hospital discharge summary). Your coverage typically begins the first day of the month after you enroll, though for ESRD it may begin the month you start dialysis or have a transplant, depending on when you enroll.

What happens if you are still working at 65

Working past 65 does not prevent you from enrolling in Medicare. If your employer has 20 or more employees and offers health insurance, you can stay on your employer plan and delay Medicare Part B without penalty — but you must enroll in Part A. If your employer has fewer than 20 employees, you should enroll in Part B when you turn 65, because your employer plan becomes secondary to Medicare.

If you delay Part B while working, you must enroll during your Initial Enrollment Period or within eight months of leaving your job or losing employer coverage. If you miss this window, you will owe a permanent 10 percent premium increase for each year you delayed, for as long as you have Medicare. This penalty does not explore to Part A, only Part B.

Automatic enrollment and what you must do yourself

If you are already receiving Social Security retirement or disability benefits when you turn 65, Medicare Part A and Part B enroll you automatically. You will receive your Medicare card in the mail about two weeks before your coverage starts. You do not have to do anything for Part A or Part B, but you still must choose a Part D plan if you want prescription drug coverage.

If you are not yet receiving Social Security when you turn 65, you must enroll in Medicare yourself. You can do this online at Medicare.gov, by phone at 1-800-MEDICARE, or in person at your local Social Security office. Even if you enroll automatically in Part A and Part B, you must actively choose a Part D plan during your enrollment period — it does not happen automatically.

Enrollment periods and what to do if you miss the important date

Your Initial Enrollment Period is the main window to enroll without penalty. It runs three months before, the month of, and three months after the month you turn 65 (or the month your disability or ESRD diagnosis begins). If you miss this window, you enter a General Enrollment Period that runs January 1 to March 31 each year, with coverage starting July 1. During General Enrollment, you can enroll in Part A and Part B, but not Part D.

If you miss your Part D enrollment period, you can enroll during the General Enrollment Period, but you will owe a penalty equal to 1 percent of the national average Part D premium for each month you were without coverage. This penalty is permanent and added to your monthly premium for as long as you have Part D. The only way to avoid this penalty is to have had creditable coverage (drug coverage as good as Medicare's) from another source during the gap.

Frequently Asked Questions

Do I have to enroll in all four parts of Medicare?

No. Part A (hospital insurance) is automatic for most people 65 and older. Part B (medical insurance) is optional but recommended for most people. Part D (prescription drugs) is optional but carries a penalty if you delay. Part C (Medicare Advantage) is an alternative to Parts A and B, not an addition — you choose one or the other, not both.

What if I am still working and my employer offers health insurance?

If your employer has 20 or more employees, you can stay on your employer plan and delay Part B without penalty. You must still enroll in Part A. If your employer has fewer than 20 employees, enroll in Part B when you turn 65, because Medicare becomes primary. When you leave your job, you have eight months to enroll in Part B without penalty.

Can I enroll in Medicare before I turn 65?

You can enroll starting three months before the month you turn 65. Your coverage will not start until the month you turn 65 (or the first of that month). If you are under 65 with a disability, ESRD, or ALS, you can enroll as soon as you meet the requirements — there is no waiting period for ESRD or ALS, but disability requires 24 months on SSDI or SSI.

What if I do not have 40 Social Security credits?

If you do not have 40 credits yourself, you may be able to enroll based on your spouse's work history (if you are 62 or older and married, or divorced after 10 years of marriage). Contact Social Security to find out whether you may have access to. If you do not may have access to for Part A, you can still buy it if you are 65 or older, though the premium is higher than for those who earned it.

When does my Medicare coverage actually start?

For people turning 65, coverage typically starts the first day of the month you turn 65. If you enroll during your Initial Enrollment Period but after the month you turn 65, there may be a delay — enroll as early as possible in your three-month window. For people with ESRD or ALS, coverage usually starts the first day of the month after you enroll, though ESRD coverage may start the month dialysis or transplant begins.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.