Medicare Basics

Medicare is an alphabet with deadlines. Learn four letters, two windows, and one trap, and the system mostly behaves.

The Four Letters

The 2026 numbers above come from Medicare directly.⁠[1] They move every year, and the Part B premium is normally deducted straight from your Social Security check, which is why the COLA and the premium increase tend to arrive in the same envelope.

The real fork is Original Medicare (A+B) + Medigap + D — broadest provider access, more predictable costs, higher monthly premiums — versus Advantage (C) — cheaper monthly, narrower networks, more administrative friction. It is close to a one-way door: your Medigap open enrollment period is the six months beginning the month you are both 65 and enrolled in Part B, and during it insurers cannot refuse you or price you on your health. Apply outside that window and, in most states, a Medigap insurer can medically underwrite you and decline.⁠[2] That makes the first choice unusually sticky, and it is worth making deliberately rather than by default.

The Two Windows and the Trap

• Initial enrollment: 7 months around your 65th birthday — the 3 months before, your birthday month, and the 3 months after.⁠[3]
• Still working at 65 with coverage from a current employer of 20 or more employees? You can usually delay B and D penalty-free, then use the special enrollment period, which runs for 8 months after the employment or the group coverage ends.⁠[3]
• The trap: COBRA and retiree coverage do not count as current-employment coverage. Neither does VA care, and neither does a spouse’s retiree plan. Misreading this buys a lifelong penalty — Part B costs an extra 10% for each full 12-month period you could have had it and did not, for as long as you have Medicare.⁠[4]
• A still-working spouse is the exception, and it is a big one. The test is not whose name is on the policy or how old either of you is — it is whether the coverage comes from somebody’s current employment. If a younger spouse is actively working and covers you as a dependent on her employer’s group plan, and that employer has 20 or more employees, that is current-employment coverage: you can delay Part B without penalty and pick up the special enrollment period when she retires or the coverage ends. The word doing the work in the line above is retiree. A spouse’s active plan protects you; a spouse’s retiree plan does not. Two cautions: employers with fewer than 20 employees flip the order, so Medicare becomes primary and you need Part B at 65 regardless; and if that plan is a high-deductible plan paired with an HSA, enrolling in Part A alone ends your HSA eligibility. Confirm your specific plan’s size and coordination rules in writing with its benefits administrator before you delay anything.⁠[3]
• HSA warning: Medicare enrollment ends HSA contribution eligibility. Stop contributions the month coverage starts — and note that claiming Social Security after 65 back-dates Part A up to six months, which can retroactively disqualify contributions you already made. The HSA triple advantage then flips to its payout phase, where the money remains available tax-free for qualified medical costs including Medicare premiums. That is the payoff of treating the HSA as a long-term account rather than a spending account: an expense you paid out of pocket years ago and never reimbursed can still be reimbursed tax-free later, for as long as you keep the receipt. The HSA Contribution & Reimbursement Log tracks contributions against the annual limit and banks those unreimbursed receipts; the Medical Claim & EOB Tracker is where the underlying statements get reconciled.

If You Already Have Coverage: VA, Military and Federal Retirees

Three large groups arrive at 65 already covered, and all three have special rules. The rules are different from each other, and the mistake they share is assuming that existing coverage makes Medicare optional. Usually it does not.

Veterans with VA health care

VA coverage and Medicare do not coordinate. They are two separate systems: Medicare does not pay for care you receive at a VA facility, and VA does not pay your Medicare cost-sharing. VA also will not cover care at a non-VA hospital unless it has authorized that care in advance.⁠[5]

Two consequences follow, and the VA itself states both. First, VA health care is not creditable coverage for Part B — it does not qualify you for a special enrollment period, so delaying Part B on the strength of VA benefits buys the permanent 10%-per-year penalty. Second, VA explicitly encourages veterans to enroll in Medicare when first eligible anyway, for the practical reason that Medicare buys access to non-VA hospitals and doctors, and for the blunter reason that VA eligibility and funding can change.⁠[6]

The one piece of good news: VA prescription drug coverage is creditable for Part D, so a veteran using the VA pharmacy can skip Part D without a penalty, as long as they enroll within 63 days of losing that coverage.⁠[6]

Military retirees on TRICARE

This one is not a suggestion. If you are eligible for both TRICARE and premium-free Medicare Part A, you must enroll in Part B to keep TRICARE. Miss it and TRICARE coverage ends.⁠[7]

Enroll on time and TRICARE converts automatically to TRICARE For Life, which works as a wraparound: Medicare pays first, TRICARE For Life picks up most of what Medicare leaves. There is no additional premium for TRICARE For Life beyond the Part B premium you are already paying, and no enrollment form — it starts when Part A and Part B are both in place and DEERS is current. TRICARE pharmacy coverage is creditable, so Part D is not required.⁠[7]

The exception worth knowing: if your sponsor is still on active duty, Part B can be delayed without penalty. The rest of the military benefits picture is in Military Money.

Federal and postal retirees

Federal retirees keep FEHB into retirement, and for most of them Part B is genuinely a choice rather than a requirement. Enroll and Medicare becomes primary with FEHB as secondary, which typically means very low out-of-pocket costs and is why many plans now offer premium rebates or reduced cost-sharing to members who take Part B.⁠[8] Decline it and FEHB simply stays primary — which is a legitimate choice, but it is close to permanent, because picking up Part B later means paying the 10%-per-year penalty for the rest of your life. FEHB drug coverage is creditable, so Part D is not needed for penalty purposes.

Postal retirees are the exception, and it is a hard rule. Under the Postal Service Health Benefits program, annuitants who retired on or after January 1, 2025 must enroll in Part B to keep PSHB coverage in retirement, with narrow exceptions. Fail to enroll when eligible and you lose PSHB eligibility permanently — enrolling in Part B later does not restore it.⁠[9]

The Annual Habit

Plans reshuffle formularies, networks and costs every year, and the plan that was right in 2024 may not be right now. Two dates:

• October 15 – December 7: Medicare open enrollment. Change Part D plans, switch between Original Medicare and Advantage, or change Advantage plans. Spend one October hour comparing your actual prescription list at Medicare.gov plan compare.
• January 1 – March 31: Medicare Advantage open enrollment — if you are already in an Advantage plan, one chance to switch plans or return to Original Medicare. This is also the general enrollment period for anyone who missed their Part B window entirely.⁠[3]

Two standing rules for the whole process: never buy supplemental coverage from a cold call, and use the free unbiased humans that already exist — your State Health Insurance Assistance Program (SHIP) counselors are trained, free, and do not sell anything.