Although you’re eligible for Medicare at 65, there are circumstances in which you might not want to apply, particularly if you’re working for a larger employer or contributing to a health savings account (HSA). However, you may face penalties if you don’t sign up at the right time, so it’s important to know when you can delay signing up without a penalty.

You can first sign up for Medicare during your Initial Enrollment Period, which is the seven-month period that includes the three months before the month you become eligible (usually age 65), the month you are eligible and the following three months. If you don’t sign up for Medicare Part B during this period, your Part B premium may go up 10 percent for each 12-month period that you were eligible but declined to have Part B. Your Medicare Part D premium will increase at least 1 percent for every month you wait. Some people who are still working may be exempt from these penalties.

If you work for an employer with 20 or more employees, you usually can delay signing up for Medicare Part B without penalty because your employer’s insurance will be considered the primary insurer. However, if your employer has fewer than 20 employees, you will probably need to sign up for Medicare Part B when you’re first eligible. Check with your employer to make sure your insurer will expect Medicare to be your primary insurer.

Note that COBRA coverage, insurance plans you purchase privately, retiree coverage and VA benefits don’t count as health insurance plans for Medicare purposes.

In other words, just that you have some type of health insurance doesn’t mean you don’t have to sign up for Part B when first eligible. To be able to delay taking Part B without penalty, your insurance must be from an employer where you actively work, subject to the 20-employee rule noted above. Before making a decision about Part B, you should contact Social Security by dialing 800-772-1213 or visiting your local office.

If you’re working or have private insurance, you may be able to delay Medicare Part D without a penalty. Beneficiaries are exempt from the penalties if their insurance is at least as good as Medicare’s. This is called “creditable coverage.” Your insurer should let you know if their coverage will be considered creditable. You may also be able to avoid or delay getting Part D if you enroll in a Medicare Advantage plan that offers prescription drug coverage.

If you’re working, you generally can enroll in Medicare Part A, which is free for most people, without consequences. However, if you are contributing to a health savings account at work, you cannot sign up for Medicare.

Part A covers institutional care in hospitals and skilled nursing facilities, as well as certain care given by home health agencies and hospice care, so you should analyze whether the HSA is worth losing out on the Part A coverage. If you’re already receiving Social Security, you will automatically be enrolled in Part A and will have to stop contributing to the HSA.