Medicare & You Handbook (2019)
Medicare - Excerpt from AARP “9 Medicare Enrollment Facts You Need to Know”
When you should sign up for Medicare — at the right time for you
Knowing when to enroll is critical, because there's no single "right" time. It depends entirely on your situation:
Initial enrollment period (IEP) at 65: This is the right time for you if you won't have health coverage from active employment (either your own or your spouse's) after you turn 65 - even if you get retiree benefits or COBRA coverage. The IEP lasts for seven months, with the fourth month usually being the one in which you turn 65. (For example, if your 65th birthday is in June, your IEP begins March 1 and ends Sept. 30). However, if your 65th birthday falls on the first day of the month, your whole IEP moves forward. (In this case, if your birthday is June 1, your IEP begins Feb. 1 and ends Aug. 31)
Initial enrollment period under age 65: If you qualify for Medicare through disability, the fourth month of your IEP is usually the one in which you receive your 25th disability payment. Social Security will let you know when your Medicare coverage starts. You get a second seven-month IEP when you turn 65 and become eligible for Medicare based on age instead of disability — but your coverage continues automatically, without your having to reapply.
Special enrollment period (SEP): This is for you if you delayed Medicare enrollment after 65 because you had health insurance from an employer for whom you or your spouse was still actively working. The SEP allows you to sign up for Medicare without risking late penalties at any time before this employment ends and for up to eight months afterward. (However, a small employer with fewer than 20 workers can legally require you to sign up for Medicare at age 65 as a condition for continuing to cover you under the employer health plan — in which case, Medicare becomes your primary insurance and the employer plan is secondary. But this decision is up to the employer, so you need to check it out before you turn 65.
**Note that if you are still working and have insurance from your employer in the form of a health savings account, under IRS rules you cannot contribute to your HSA if you are enrolled in any part of Medicare. In this situation you need to postpone signing up for Part A and Part B until you retire and also postpone applying for Social Security (because you can't opt out of Part A if you're receiving those benefits). You won't be penalized for this delay.