Turning 65 does not come with a single deadline. It comes with a set of windows, and which one applies depends on whether you are still working, how many employees your employer has, and whether you are already drawing Social Security. Get the window wrong and the cost is not a one-off fee. Medicare states plainly that late enrollment penalties are added to the monthly premium, are not a one-time late fee, and for most people last a lifetime.
This is a timing problem, not a shopping problem. The plan choice can be revisited every year. The enrollment date cannot.
The initial window: seven months, and the middle of it is not the safe part
The Initial Enrollment Period runs from three months before the month you turn 65 through three months after it. Seven months in total.
The instinct is to file in the birthday month. The better move for anyone who wants coverage active on day one is to file in the first three months, because coverage timing depends on when Social Security processes the application rather than on the birthday itself. Filing early costs nothing and removes the gap risk.
Premium-free Part A is a work-history test, not an age test
About 99 in 100 beneficiaries never see a Part A bill, on the CMS count. The qualifying test is 40 quarters of Medicare-covered employment, credited to you or a spouse and verified by the Social Security Administration. That is ten years of covered work, and the quarters need not run consecutively.
Short of that, Part A becomes something you buy. The 2026 price is $311 monthly for anyone credited with 30 to 39 quarters, and $565 monthly below 30. Anybody in that position should check their earnings record with Social Security well before 65, because a few missing quarters can sometimes still be earned.
The Part A penalty also works differently from the others. Medicare states that if you have to buy Part A and do not enroll when first eligible, the premium may rise 10 percent, and that you pay the higher premium for twice the number of years you did not sign up. It ends. The Part B penalty generally does not.
The Part B penalty, in the numbers Medicare publishes
Medicare charges an extra 10 percent of the standard Part B premium for each full 12-month period somebody could have signed up and did not, and it is generally charged for as long as they have Part B.
Medicare’s own worked example for 2026: somebody who waited two full years and did not qualify for a Special Enrollment Period pays a 20 percent penalty on the standard $202.90 premium. That is $40.58 added, for a monthly Part B premium of $243.50 after rounding. Not for two years. For the rest of their life, rising each year as the standard premium rises.
Working past 65: the employee count decides everything
The single question that determines whether somebody can safely delay Part B is how many people the employer has. Medicare’s own eligibility tool asks it directly.
| Situation at 65 | What Medicare says to do | Why |
|---|---|---|
| Still working, employer has 20 or more employees | Part A is usually worth taking. Part B can generally wait until you or your spouse stop working. | Job-based coverage pays first, and an 8-month Special Enrollment Period protects you afterwards. |
| Still working, employer has fewer than 20 employees | Check with the employer and generally sign up for both A and B at 65. | Medicare pays first in small-employer groups, so the job-based plan may not pay for services without Part B in place. |
| Retired, on retiree or COBRA coverage | Sign up at 65. | Neither counts as active job-based coverage for the Special Enrollment Period. |
| On a Marketplace plan | Sign up when first eligible and end the Marketplace plan. | Once eligible for premium-free Part A you no longer qualify for Marketplace premium help, and keeping it can mean repaying that help at tax time. |
Row three is where most penalties are actually earned. COBRA feels like employer coverage and is not treated as such. Medicare is explicit that the 8-month Special Enrollment Period starts when you stop working, even if you choose COBRA or other non-Medicare coverage. Somebody who takes 18 months of COBRA at 65 and assumes they are covered will exhaust the Medicare window ten months before the COBRA ends.
One more trap for people still working: contributions to a Health Savings Account must stop once Medicare begins, and Part A enrollment can be backdated. Anyone contributing to an HSA at 65 should get the timing checked before filing rather than after.
The window nobody mentions until it has closed
Medigap supplemental policies have their own timing, and it is separate from everything above. The federal open enrollment window for Medigap is six months beginning when somebody is 65 and enrolled in Part B. Inside it, insurers must sell regardless of health.
Outside it, and outside the states whose own laws are more generous, an insurer may medically underwrite and may decline. That is why the decision at 65 between Original Medicare with a supplement and a Medicare Advantage plan is less reversible than the annual open enrollment period makes it look. Switching from Advantage back to Original Medicare is permitted; buying the supplement that makes Original Medicare workable may not be, years later, on the same terms.
State rules vary here more than on almost any other point in Medicare, so the state insurance department is the authority worth checking rather than any national summary, including this one.
What to do, in order, from about age 64 and a half
- Check the earnings record with Social Security. Confirm 40 quarters. This is the difference between a free Part A and $565 a month.
- Establish which window applies. Still working with 20 or more employees is the only common case where delaying Part B is safe. Get the employee count from HR, not from an assumption.
- Get the creditable-coverage letter in writing. Both the health and the drug side. It is the evidence that defeats a penalty claim later, and nobody keeps a copy for you.
- File in the first three months of the window, not the birthday month, if you want coverage running without a gap.
- Decide the delivery question before the Medigap window closes, because that six-month clock starts with Part B and does not reopen.
Nothing on this page is a recommendation about a plan, and no plan or carrier is named anywhere on this site. The figures come from CMS and Medicare.gov. Free unbiased help is available from the State Health Insurance Assistance Program in every state, and those counsellors are not paid a commission on what you choose.
Questions readers actually ask
Do I have to sign up at 65 if I am still working?
It depends on the size of the employer. With 20 or more employees the job-based plan generally pays first and Part B can wait, protected by an 8-month Special Enrollment Period after the work ends. With fewer than 20 employees Medicare generally pays first, so the job plan may not cover services without Part B in place. Confirm the employee count with the employer.
Does COBRA let me delay Part B?
No. Medicare states the 8-month Special Enrollment Period starts when you stop working, even if you choose COBRA or other coverage that is not Medicare. COBRA lasting longer than that window does not extend it, and this is one of the most common ways people end up with a permanent penalty.
How much does waiting actually cost?
Medicare’s published 2026 example: two full years without Part B and without a Special Enrollment Period produces a 20 percent penalty, taking the standard $202.90 premium to $243.50 a month. That surcharge generally continues for as long as somebody has Part B, and it is recalculated as the standard premium changes.
Can I change my mind later?
Partly. Drug and Advantage plan choices can be revisited each year in the autumn open enrollment. The Medigap decision is the one with a closing door: the federal guaranteed-issue window is six months from when you are 65 and enrolled in Part B, and after that insurers may medically underwrite unless your state requires otherwise.
Sources
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Every figure in this article traces to a government record or to a named independent, non-commercial research body. We do not cite insurance marketplaces or affiliate comparison sites for data.
- Centers for Medicare & Medicaid Services, Medicare.gov Avoid late enrollment penalties Published 2026-01-01Supports: Late enrollment penalties are added to the monthly premium, are not a one-time late fee, and are usually charged for as long as you have that type of coverage; Part A penalty raises the premium 10 percent and is paid for twice the number of years not signed up; Part B penalty is an extra 10 percent for each full 12-month period, with the 2026 worked example of a 20 percent penalty on the $202.90 standard premium giving $40.58 and a total of $243.50; Part D penalty is 1 percent per month (12 percent a year) after going 63 days or more without creditable drug coverage, calculated on the national base beneficiary premium of $38.99 in 2026, with the worked example of 14 months giving $5.50 a month, added for as long as you have Medicare drug coverage even if you switch plans; no Part D penalty with creditable coverage or Extra Help.
- Centers for Medicare & Medicaid Services, Medicare.gov When can I sign up for Medicare? Published 2026-01-01Supports: Initial eligibility runs from 3 months before the month you turn 65 to 3 months after it; with employer coverage from an employer of 20 or more employees, Part A may be taken at 65 and Part B may wait, with an 8-month Special Enrollment Period after work stops; the 8-month period starts when you stop working even if you choose COBRA or other non-Medicare coverage; where the employer has fewer than 20 employees the job-based coverage might not pay without both Part A and Part B; Part B coverage during the SEP starts the month after SSA or the RRB receives completed forms and an extra form showing job-based coverage is required; sign up for Part B the month before retiring to avoid a gap; a 100-employee threshold applies for coverage through a family member where Medicare is held due to disability or ALS; Medicare eligibility ends Marketplace premium help and keeping it may require repayment at tax time; state Medicaid programs in most cases pay the Part B premium.
- Centers for Medicare & Medicaid Services 2026 Medicare Parts A & B Premiums and Deductibles Published 2025-11-14Supports: Roughly 99 percent of beneficiaries pay no Part A premium having at least 40 quarters of Medicare-covered employment as determined by SSA; 2026 Part A buy-in premiums of $311 with 30 to 39 quarters and $565 with fewer than 30; 2026 Part B standard premium $202.90.
Figures last verified August 30, 2026.