06/05/2026
Working past 65 at a company that still carries solid health insurance, the standard advice in HR offices and at kitchen tables runs about the same: enroll in Medicare Part A because it's free, but skip Part B until the employer plan ends. The advice is correct for a specific federal-rule reason that most HR conversations don't name out loud.
Here's the federal rule that drives the decision.
Medicare Part A is hospital insurance. For workers with 40 or more quarters of covered employment (10 years of work with F**A withholding), Part A has no monthly premium - Medicare treats Part A as earned through the worker's payroll-tax history. Part B is medical insurance and has a monthly premium ($202.90 standard for 2026).
When a worker stays on a group health plan from a current employer with 20 or more employees AND continues working past 65, the federal rule sets that employer plan as the PRIMARY payer for healthcare claims, and Medicare as the SECONDARY payer. The federal authority is 42 CFR section 411.172 (Medicare secondary payer rules for working aged individuals). The employer plan pays first. Medicare considers covering whatever's left after the employer plan's processing.
Part A enrollment in this situation is free and can coordinate with the employer plan - the employer plan still pays primary on most claims, and Part A acts as a secondary backstop on inpatient hospital costs. Most workers in this scenario do enroll in Part A simply because there's no premium and no downside.
Part B enrollment in this situation is different. The Part B premium continues monthly regardless of how the federal coordination works - meaning the worker would pay $202.90 per month for Medicare Part B coverage that the employer plan is effectively duplicating. Skipping Part B until the employer coverage ends, then using the federal Special Enrollment Period (8-month window starting the month after employer coverage ends), avoids paying for redundant coverage AND avoids the late-enrollment penalty.
The trap most HR offices don't mention: enrolling in Part A also disqualifies HSA contributions. The federal HSA rule, codified at IRC section 223(b)(7), requires that the account holder NOT be enrolled in any part of Medicare. Once Part A starts, the federal HSA-eligibility rule turns off. Contributions made during a month of Part A enrollment - whether through payroll deduction or personal contribution - become excess contributions, subject to a 6% federal excise tax per year on the excess until removed.
The federal carve-out for HSA-using workers: a worker who would lose HSA eligibility by enrolling in Part A can SKIP Part A enrollment and continue HSA contributions. The federal rule allows the worker to refuse Part A as long as the worker hasn't yet filed for Social Security retirement benefits AND hasn't yet enrolled in any other part of Medicare. (Filing for Social Security at or after 65 automatically enrolls the worker in Part A, with the retroactive 6-month back-date trap - that's a separate federal mechanic.)
A 67-year-old working at a company with 200 employees, contributing $4,300/year to an HSA through payroll, taking advantage of the company's high-deductible health plan - the federal-rule-aware move is to DELAY both Part A and Part B until the federal Special Enrollment Period opens at the worker's retirement. The HSA contributions continue. The 8-month SEP window starts the month after employer coverage ends, regardless of whether the worker is 67, 68, 70, or older when retiring.
The federal coordination calculation works year-by-year. A worker who initially skipped Part A and now plans to retire and start Social Security at the same time needs to plan the calendar around the retroactive Part A enrollment trap - stop HSA contributions 6 months before filing for Social Security, then file.