The 20 employee line is the rule that decides almost everything here, and it is worth confirming with your benefits administrator rather than guessing.
Employers with 20 or more employees. Your group coverage is primary and Medicare is secondary. Most people in this position enroll in premium free Part A and delay Part B until the job or the coverage ends. When it does, an eight month Special Enrollment Period opens for Part B, with no late penalty.
Employers with fewer than 20 employees. Medicare generally becomes primary at 65. If you have not enrolled in Part B, the group plan may pay only what it would have paid as secondary, which can leave large balances. In this situation most people enroll in both Part A and Part B on time.
The HSA trap. You cannot contribute to a health savings account once you are enrolled in any part of Medicare. Part A enrollment is also backdated up to six months when you sign up after 65, so contributions made in that retroactive window can become excess contributions. If you plan to keep contributing to an HSA, stop contributions at least six months before you enroll.
COBRA is not active employment. COBRA and retiree coverage do not count as employer coverage for delaying Part B. Relying on either past 65 is a common and expensive mistake.
Drug coverage. Separately, confirm in writing whether the employer drug coverage is creditable for Part D. Keep the annual notice, because it is what protects you from a Part D penalty later.
Related on this site: Turning 65 in Idaho checklist · Medicare resources
Sources
- Working past 65 (CMS) · accessed September 14, 2026
- Part B late enrollment penalty (CMS) · accessed September 14, 2026
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