I'm 65 and still have insurance through my job. Can I wait to sign up for Medicare Part B without a penalty?

Medicare.gov says there are situations after your first chance to sign up when you can join Part B without a late penalty [1]. One is having health insurance through your job, or your spouse's job [1]. The federal rule adds that the coverage must be because of your current work, or your spouse's current work [2].

This Special Enrollment Period starts the first month after your first sign-up period ends. It ends 8 months after the job coverage ends or the job ends, whichever comes first [1]. Coverage usually starts the first month after you sign up. If you sign up while you still work, or in the first full month after job coverage ends, you can ask to delay the Part B start date up to 3 months [1].

COBRA is not counted as job coverage. Getting COBRA does not change when this period ends [1]. The page also says this period does not apply if you qualify for Medicare because of kidney failure (ESRD) [1]. The sources here do not say what a retiree plan or a small employer's plan does with Part B: "Don't know yet".

Open Medicare.gov, When does Medicare coverage start?, and read the Special Enrollment Period part for job coverage. Your own plan papers hold details these sources do not give: "Don't know yet" for your case.