Dear Savvy Senior,

Should I enroll in Medicare at age 65 if I’m still working and have coverage through my employer?

Almost 65

Dear Almost,

The rules for enrolling in Medicare can be very confusing with all the different choices available today. But when you postpone retirement past age 65, as many people are doing, it becomes even more complicated.

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. And Part B, which covers doctor’s bills, lab tests and outpatient care. Part B also has a monthly premium of $104.90 in 2016 (though it’s higher for individuals earning $85,000 or more a year).

If you are receiving Social Security, you will be enrolled automatically in parts A and B when you turn 65. If you aren’t yet receiving Social Security, you will have to apply, which you can do online at SSA.gov/medicare, over the phone at 800-772-1213 or through your local Social Security office.

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. But to decide whether to take Part B or not will depend on the size of your employer.

Small employer: If your current employer (or spouse’s employer if it’s providing your coverage) 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 65^th 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 percent penalty for each year you wait beyond your initial enrollment period, which will be tacked on to 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 (or your spouse if the coverage is from his/her employer) 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.

Drug coverage: You also need to verify your prescription drug coverage. Call your benefits manager or insurance company to find out if your employer’s prescription drug coverage is considered “creditable.” (Creditable prescription drug coverage is one that is considered to be as good as or better than the Medicare prescription drug benefit.) 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/find-a-plan) during your initial enrollment period or you’ll incur a premium penalty (1 percent of the average national premium for every month you don’t have coverage) if you enroll later.

For additional help, visit Medicare.gov or contact your State Health Insurance Assistance Program (SHIP) at Shiptacenter.org. The Medicare Rights Center also offers a free helpline at 800-333-4114.

Send your senior questions to: Savvy Senior, P.O. Box 5443, Norman, OK 73070, or visit SavvySenior.org. Jim Miller is a contributor to the NBC Today show and author of The Savvy Senior book.