Medicare at 65: The Enrollment Mistakes That Cost Retirees Thousands

I turned 65 in October. Two months before, I got a thick envelope from Medicare and I did what most people do: I filed it in the stack with the other insurance paperwork and figured I had more time.

I almost made a $4,000 mistake.

Here is what I learned — the hard way, through research and conversations with people who have been there — about Medicare enrollment windows, late penalties, and the choice that will define your healthcare for the next 20 years.

The 7-Month Window Nobody Explains Clearly

When you turn 65, you get a 7-month Initial Enrollment Period (IEP). It starts 3 months before your 65th birthday month and ends 3 months after.

During those 7 months, you can enroll in Medicare Part A (hospital insurance), Part B (medical insurance), and Part D (prescription drug coverage). You can also sign up for a Medicare Advantage plan (Part C) that bundles these together.

Here is the part that trips people up: Part A is usually free if you or your spouse paid Medicare taxes for 10+ years. Most people sign up for Part A automatically and think they are done.

But Part B — which covers doctor visits, outpatient care, preventive services — you have to actively sign up for. And if you do not sign up during your IEP, and you do not have creditable coverage through an employer, you face a late enrollment penalty that lasts for life.

The Late Penalty Is Not Small

The Part B late enrollment penalty is 10% of your monthly premium for every 12-month period you could have been enrolled but were not.

Let me make that concrete. Say you miss your IEP entirely and enroll 2 years late. That is a 20% penalty added to your Part B premium — permanently. If your Part B premium is $175/month (the standard 2025 rate), you are now paying $210/month extra. Over 20 years, that is $50,000 you do not need to spend.

The penalty resets if you go 12 consecutive months without Part B coverage. So if you have employer coverage at 65, leave your job at 67, and then enroll — you may face a penalty based on how long you went without coverage.

This is why timing matters so much.

Credible Coverage: The One Exception

If you are still working at 65 and on an employer health plan with 20+ employees, you can delay Part B without penalty. Your IEP clock starts when that employer coverage ends.

The rule: the employer plan must be current and active — not COBRA, not a retiree plan from a former employer that you are paying for yourself. If you are unsure, ask your HR department to confirm in writing.

Once you retire, you get an 8-month Special Enrollment Period (SEP) to sign up for Part B without penalty.

Medigap vs. Medicare Advantage: The Choice Most People Rush

After you enroll in original Medicare (Parts A and B), you will be deluged with mail about Medicare Advantage plans. They advertise $0 premiums, dental coverage, vision, gym memberships. It sounds great.

Here is what those mailers do not say: Medicare Advantage plans are HMOs and PPOs. You must use their network of doctors. If you want to see a specialist, you often need a referral. If your doctor is not in the network, you pay full price unless it is an emergency.

Original Medicare plus a Medigap policy (supplemental insurance that covers what Medicare does not — deductibles, copays, coinsurance) gives you the freedom to see any doctor who accepts Medicare (and 99% do). The Medigap premium runs $100-$300/month depending on your plan and location, but it eliminates the unpredictable out-of-pocket costs that original Medicare leaves you exposed to.

The critical window: You have 6 months after first enrolling in Part B to buy any Medigap policy with guaranteed acceptance — no medical questions, no health exclusions. After that window closes, insurers can deny coverage or charge higher premiums based on your health.

This is why the Medigap decision made in your first 6 months of Part B matters so much. People who wait until they have health problems to buy Medigap often cannot get it or can only get it at much higher cost.

Part D: Prescription Coverage You Cannot Ignore

Medicare Part D covers prescription drugs. If you do not enroll in Part D or a Medicare Advantage plan with drug coverage during your IEP, you face a late enrollment penalty similar to Part B.

The penalty is 1% of the "national base beneficiary premium" ($35.52/month in 2025) for every month you went without creditable drug coverage. Over time, that compounds.

If you are on expensive medications, check the Part D plans available in your zip code. The coverage and costs vary significantly between plans — one that covers your specific medications may save you hundreds of dollars per month versus the cheapest option.

What I Did

I enrolled in Part A (free, automatic), Part B (actively signed up), and a Part D plan that covers my blood pressure medication and the new prescription my doctor mentioned for cholesterol. I did not go with Medicare Advantage because I wanted the flexibility to see any doctor without referrals — I have a cardiologist I have been with for 12 years and I was not going to lose that.

Total monthly cost for my coverage in 2025: Part B premium ($174.70) + Part D plan ($12/month) + Medigap Plan G ($215/month) = roughly $400/month. My employer plan at 65 would have been $600/month for worse coverage. The math was not complicated.

The enrollment process: Go to ssa.gov/medicare, create a My Social Security account, and enroll online. It takes about 30 minutes. Do not leave it to the last month.

Do not be like me and almost file it in the stack.

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This is Matthew's personal experience. Medicare rules change annually — verify current rates and deadlines at Medicare.gov. This article contains affiliate links to insurance information resources.