If you work for an employer with 20 or more employees and have health coverage through that job, you can delay Medicare Part B without a penalty for as long as you keep working. If the employer has fewer than 20 employees, Medicare becomes your primary insurance at 65 and you need to enroll. That headcount is the whole rule.
Large employer: 20 or more
Your group plan pays first and Medicare pays second, so most people in this position take Part A, which is free, and skip Part B until they retire. Skipping Part B saves $202.90 a month in 2026.
When you stop working or lose the coverage, you get a Special Enrollment Period of eight months to sign up for Part B with no penalty. Sign up in the first month or two so there is no gap.
One exception: if your job coverage is a Health Savings Account plan and you want to keep contributing to the HSA, do not enroll in any part of Medicare, including Part A. Medicare enrollment ends HSA contributions. Stop contributing six months before you plan to enroll, because Part A is backdated up to six months.
Small employer: fewer than 20
Medicare pays first, and your group plan may pay very little or nothing until you enroll. Enroll in Parts A and B during your Initial Enrollment Period. Compare what your group plan costs and covers against a Medicare plan; many people find Medicare with a Supplement is both cheaper and better.
Retiree coverage and COBRA do not count
Retiree plans, COBRA and marketplace plans are not active employer coverage. They do not let you delay Part B without a penalty. If you are on any of these when you turn 65, enroll on time.
Spouse's coverage
If you are covered through your spouse's employer and that employer has 20 or more employees, the same delay rule applies to you.
Part D while working
If your employer plan's drug coverage is at least as good as Medicare's, called creditable coverage, you can delay Part D without a penalty. Your plan sends a notice each year saying whether it is creditable. Keep that letter.
Get this wrong and the penalties last for life. Get it right and you save money for years. Request a plan check, tell me about your job coverage, and I map out your dates.