Ask the Expert: Don't miss Medicare deadline

I won't take Social Security until I'm 66, but I know I have to sign up for Medicare at 65. How do I pay for it if I'm not collecting Social Security?

 You'll be billed every three months for Medicare Part B, which covers doctors' visits. (If you don't get a bill by the 10th of the month, call Social Security.) Medicare Part A, which covers hospital care, is free.

Mark your calendar to apply for Medicare three months before your 65th birthday! You won't get an enrollment notice. People already collecting Social Security are automatically enrolled in Medicare at age 65. Everyone else must call Social Security at 800-772-1213 to sign up for it.

The initial Medicare enrollment period starts three months before your 65th birthday and ends three months after your birthday - but if you want Part B coverage to start the month you turn 65, you must apply in the three months before your 65th birthday.

If you wait until a couple of months after your birthday, your Part B coverage won't begin until three months after you enroll. That could leave you temporarily without primary coverage if your private health insurance becomes secondary when you turn 65, as many policies do.

If you miss initial enrollment, you can sign up within eight months of your last day at work or the day your employer-sponsored coverage ends, whichever comes first. If you miss that deadline, your Part B premium is permanently higher. And that's not all: Your only remaining option is annual open enrollment between Jan. 2 and March 31, for coverage starting in July. Missing that annual window could leave you uninsured for more than a year.

The bottom line: Missing a Medicare enrollment deadline can be costly.