When should you enroll in Medicare?

The seven-month initial enrollment period

Medicare.gov describes the initial enrollment period as seven months long: it starts three months before the month you turn 65, includes your birthday month, and ends three months after it. If you already receive Social Security benefits, you are generally enrolled in Part A and Part B automatically. Everyone else has to act.

When you sign up inside that window determines your start date:

Premium-free Part A follows a friendlier rule. Coverage begins the month you turn 65, or the month before if your birthday falls on the first day of a month, and you can add it at any time later. Medicare.gov also notes that when you sign up for premium-free Part A after 65, coverage is backdated up to six months, though never earlier than the month you turned 65. That backdating is harmless for most people and expensive for HSA holders, as covered below.

Working past 65 with employer coverage

If you or your spouse are still working and covered by an employer group health plan available to all employees, Medicare.gov says you can generally wait to take Part B until the work or the coverage ends, whichever comes first, without a late enrollment penalty. Most people in that situation take premium-free Part A at 65 and delay Part B.

Two details determine whether that is safe:

• Employer size. At employers with 20 or more employees, the group plan generally pays first and delaying Part B is normally safe. At smaller employers, Medicare typically pays first, so declining Part B can leave you with almost no coverage. Ask your benefits administrator in writing which one applies.
• Whether it is really group coverage. Medicare.gov warns that if you are self-employed, or the coverage is not offered to everyone at the company, or you buy your own plan with an employer stipend, you should confirm with the insurer whether it counts as employer group health plan coverage. If it does not, sign up at 65 to avoid the Part B penalty.

Retiree coverage is not active employer coverage. Medicare.gov states that retiree coverage from a former job may not pay for services unless you have both Part A and Part B. The same is true of most TRICARE for Life arrangements. Treat retiree coverage as a supplement to Medicare, not a substitute for it.

The HSA contribution stop rule

Once you are enrolled in any part of Medicare, you may no longer contribute to a health savings account. You can still spend the balance tax-free, including on Medicare Part B, Part D and Medicare Advantage premiums, but new contributions must stop.

The trap is retroactivity. Because Medicare.gov says premium-free Part A is backdated up to six months when you enroll after 65, an enrollment in, say, October can make Part A effective back to April. Any HSA contributions made for those retroactive months become ineligible, and excess contributions are taxable and subject to penalty until corrected.

Two related points. Your employer can keep contributing to a spouse's HSA if the spouse remains covered by a qualifying high-deductible plan and is not on Medicare. And if you are staying on an employer high-deductible plan past 65 specifically to keep funding an HSA, do not enroll in Part A at all, because Part A alone ends your eligibility. Our HSA versus PPO guide covers the contribution mechanics.

What the late penalties actually cost

Both penalties are permanent surcharges on your monthly premium, not one-time fees, and both grow the longer you wait.

Part B. Medicare.gov states the penalty is an extra 10% of the standard premium for each full 12-month period you could have had Part B and did not. Using CMS figures for 2026, the standard premium is $202.90 a month:

A 20% penalty is roughly $487 a year, every year, and it rises as the base premium rises. Over a 20-year retirement that is real money for a paperwork error.

Part D. The penalty is 1% of the national base beneficiary premium for each month you went without creditable drug coverage after your initial enrollment period, triggered once you go 63 days or more without it. CMS set the 2026 national base beneficiary premium at $38.99, so 24 uncovered months costs about $9.36 a month added to whatever your drug plan charges, and the surcharge follows you even if you switch plans.

Part A. Most people pay no Part A premium at all, because CMS reports about 99% of beneficiaries have at least 40 quarters of covered employment. If you must buy Part A, the 2026 premium is $565 a month, or $311 with 30 or more quarters, and the Part A late penalty works differently from the others.

Special enrollment periods

The special enrollment period is what makes delaying Part B safe. Medicare.gov describes it as an eight-month window that begins when you stop working or lose the employer coverage, whichever happens first.

The critical warning is in the same guidance: the eight months start running from the loss of employment or coverage even if you elect COBRA. COBRA is not active employer coverage for Medicare purposes. People who take 18 months of COBRA at 65 and assume Medicare can wait routinely arrive with a Part B penalty and a coverage gap.

• Do not wait out COBRA. Enroll in Part B within the eight-month window that started when the job ended, not when COBRA ends.
• Drug coverage runs on a shorter clock. The Part D special enrollment period is generally two months after creditable drug coverage ends, and the penalty clock starts after 63 days.
• Keep the paperwork. Social Security asks for proof of group coverage, typically CMS forms L564 and 40B, so request the employer attestation before you leave the job.
• Other special enrollment periods exist. Moving out of your plan area, losing Medicaid, a plan leaving your county, or living in an area affected by a declared emergency can each open one.

The general enrollment period, your last resort

If you miss the initial enrollment period and have no special enrollment period, Medicare.gov says you can sign up during the general enrollment period, January 1 through March 31 each year, with coverage starting the month after you enroll. That is an improvement over the old rules, which delayed coverage until July, but a late-enrollment penalty generally still applies.

Sequence matters after that. You need Part A and Part B before you can buy a Medigap policy or join a Medicare Advantage plan, and your six-month Medigap open enrollment period starts the first month you have Part B and are 65 or older. Enrolling in Part B late therefore delays your one guaranteed shot at a Medigap policy without medical underwriting. Our Advantage versus Medigap comparison explains why that window is the most valuable date on the Medicare calendar.

A clean timeline for the year you turn 65

• Twelve months out. Ask your benefits administrator, in writing, whether your employer plan has 20 or more employees, whether the drug coverage is creditable, and how the plan coordinates with Medicare.
• Seven months out. If you contribute to an HSA and plan to take Medicare at 65, stop contributions now to stay clear of the six-month Part A backdating.
• Four to three months out. Create a Social Security account and enroll in Part A and Part B so coverage begins the month you turn 65. If you are keeping active employer coverage, decide whether to take Part A alone or nothing at all.
• Two months out. Choose your path: Original Medicare plus a Medigap policy and a Part D plan, or a Medicare Advantage plan. Check that your doctors and drugs are covered under each.
• Month of your 65th birthday. Your six-month Medigap open enrollment window begins once Part B is in force. This is the only period when insurers in most states must sell you any Medigap policy regardless of health.
• When you retire later. File for Part B within eight months of leaving the job, before COBRA runs out, and enroll in a Part D plan within two months of losing creditable drug coverage.

Enrollment rules are federal, but Medigap availability, underwriting protections and plan choices vary by state. Confirm your own dates with Social Security or Medicare before you rely on any timeline.