• You do not get an SEP, even if you have group health insurance and plan to continue working past 65, if:
  
  (a) Your employer or union health plan automatically becomes secondary to Medicare when you (or your spouse) turn 65 — which may happen if the employer has fewer than 20 workers.
  
  (b) You are covered by COBRA temporary group health insurance.
  
  (c) You are covered by the employer insurance of your domestic partner who is the same sex as you.

In all these circumstances, the Part B penalty clock starts ticking at the beginning of the month after your IEP expires.

• If you have individual health insurance and continue to work after age 65, the penalty clock starts ticking at the beginning of the month after your IEP expires. (With individual, instead of group, insurance, you don't get an SEP if you delay joining Part B.)

• If you become entitled to Medicare at a younger age through disability, your deadline may vary according to the same circumstances explained above, with one exception: If you are covered by a group health plan provided by an employer for whom you or your spouse are working, you may be required to enroll in Part B if the employer has fewer than 100 employees. However, if you miss your deadline for signing up for Part B and incur a penalty, the clock will be reset as soon as you turn 65. You'll then be eligible for Medicare based on your age instead of disability and you will no longer have to pay the penalty.

• If you live outside the United States when you turn 65 (and if you do not have American-style employer group health insurance or coverage from an approved American volunteer program), the penalty clock starts ticking at the end of your IEP. To avoid a late penalty, you must enroll and pay Part B premiums, even though you cannot use any Medicare services while overseas. You do not get an SEP to sign up when you return to live in the United States.

Note: If your state pays your Part B premiums under a Medicare Savings Program for people with limited incomes, the state will also pay any Part B penalties you've incurred.

Also of interest: How would you strengthen Medicare?