If you missed your initial 7-month enrollment period into Original Medicare, there is a General Enrollment Period.
General Enrollment Period – If you didn’t sign up for Part B (or Part A if you need to purchase it) when you were first able during your initial enrollment period and you didn't have a special enrollment period, there is an annual General enrollment period from January 1 – March 31. Your Medicare coverage will begin July 1. Your Part B (and/or Part A) premium could be assessed a penalty, therefore it will be higher than if you enrolled during your Initial enrollment period. The penalty is 10% per every 12 months you didn't enroll when first eligible. You could be paying this penalty for as long as you're enrolled into Medicare.
If you have questions about enrolling into Medicare, call Social Security Administration at 1-800-772-1213.