from medicare.gov (http://medicare.gov/sign-up-change-plans/i-have-employer-coverage)
It’s important to understand how your current coverage works with Medicare. If you have questions about your current insurance, the best source of information is your benefits administrator, insurer, or plan provider.
I have employer coverage and…
I’m Turning 65
If you aren’t getting benefits from Social Security (or the RRB) at least 4 months before you turn 65, you’ll need to sign up with Social Security to get Parts A and B. However, depending on the size of the employer, you may be able to delay Parts A and B without having to pay a penalty if you enroll later. Learn more about whether you should get Parts A and B.
I’m Over 65
In most cases, you don’t need to do anything until you (or your spouse) retire or you lose the employer coverage. If you didn’t enroll when you were first eligible, the size of the employer determines whether you have to pay a penalty if you enroll later. Learn more about whether you should get Parts A and B and what happens when your employment or coverage ends.
I’m Under 65 And Have A Disability
In most cases, you don’t need to do anything until you (or your spouse) retire or you lose the employer coverage. If you didn’t enroll when you were first eligible, the size of the employer determines whether you have to pay a penalty if you enroll later. Learn more about whether you should get Parts A and B and what happens when your employment or coverage ends.