Missing deadlines can result in costly penalties and delays. By the editors of Kiplinger's Personal Finance Originally published December 3, 2015 Many retirees rely on Medicare. Part A, which pays for hospitalization, is free. There’s a monthly premium for Part B, which covers outpatient care, such as doctors’ visits.See Also: 10 Reasons You Will Never Retire You’re eligible for Medicare at 65. If you’re already receiving Social Security, you’ll be automatically enrolled and receive your Medicare card three months before your birthday. If not, you must enroll yourself. You can sign up for Medicare without penalty beginning three months before until three months after your 65th birthday. If neither you nor your spouse has health insurance through a current employer, sign up for both Part A and Part B. Even if you’re still working at 65, sign up for Part A. If your employer has fewer than 20 employees, sign up for Part B, too, since employee coverage generally becomes secondary to Medicare at age 65. Otherwise, you must sign up for Part B within eight months of retiring from your job. There’s a lot more you need to know about Medicare, including other common Medicare mistakes to avoid. Kiplinger's special report on Navigating Medicare can help.