Dear Savvy Senior: I will turn 65 in a few months and plan to keep working for several more years. I have good health insurance from my employer now. Do I have to sign up for Medicare when I reach 65? — Looking Ahead

Dear Looking: Whether you need to enroll in Medicare at 65 if you continue to work and have health insurance through your job depends on how large your employer is. The same rules apply if your health insurance comes from your spouse’s job.

But first, let’s review the basics. Remember that original Medicare has two parts: Part A, which provides hospital coverage and is free for most people. Part B covers doctor’s bills, lab tests and outpatient care. Part B also has a monthly premium, which is $148.50 for most beneficiaries in 2021 but is higher for individuals earning above $88,000.

If you’re already receiving Social Security, you’ll automatically be enrolled in Parts A and B when you turn 65, and you’ll receive your Medicare card in the mail. It will include instructions to return it if you have work coverage that qualifies you for late enrollment. If you aren’t yet receiving Social Security, you will have to apply, which you can do online at SSA.gov/medicare.

If you plan to continue working past the age of 65 and have health insurance from your job, your first step is to ask your benefits manager or human resources department how your employer insurance works with Medicare. In most cases you should at least take Medicare Part A because it’s free. (Note: If you’re funding a health savings account, you might not want to take Part A because you can’t make contributions after you enroll.) But to decide whether to take Part B will depend on the size of your employer.

Small employer

If your current employer has fewer than 20 employees, Medicare will be your primary insurer, and you should enroll in Medicare Part B during your initial enrollment period. This is a seven-month period that includes the three months before, the month of and the three months after your 65th birthday.

If you miss the seven- month sign-up window, you’ll have to wait until the next general enrollment period, which runs from Jan. 1 to March 31 with benefits beginning the following July 1. You’ll also incur a 10% penalty for each year you wait beyond your initial enrollment period, which will be tacked onto your monthly Part B premium.

Large employer

If your employer has 20 or more employees, your employer’s group health plan will be your primary insurer as long as you remain an active employee. If this is the case, you don’t need to enroll in Part B when you turn 65 if you’re satisfied with the coverage you are getting through your job. But if you do decide to enroll in Medicare, it will supplement your employer insurance by paying secondary on all of your claims.

Once your employment or group health coverage ends, you will then have eight months to sign up for Part B without a penalty. This is known as the special enrollment period.

Check drug coverage

You also need to verify your prescription drug coverage. Call your benefits manager or insurance company to find out whether your employer’s prescription drug coverage is considered “creditable.” If it is, you don’t need to enroll in a Medicare Part D prescription drug plan. If it isn’t, you should purchase a plan (see Medicare.gov/plan-compare) during your initial enrollment period or you’ll incur a premium penalty (1% of the average national premium for every month you don’t have coverage) if you enroll later.

If you have more questions or need help, contact your State Health Insurance Assistance Program (see ShiptaCenter.org), which offers free Medicare counseling, or call the Medicare Rights Center helpline at 800-333-4114.

Jim Miller is a contributor to NBC-TV’s “Today” program and author of “The Savvy Senior.” Send your questions to Savvy Senior, P.O. Box 5443, Norman, OK 73070; or visit savvysenior.org.