I never enroll in Medicare.
If you're not already collecting Social Security, Medicare usually does not start automatically. That is where people get into trouble.
If you're collecting Social Security
You're auto-enrolled in Part A and Part B around your 65th birthday. You can decline Part B, but Part A enrollment is generally locked in once you've claimed Social Security.
If you're not collecting Social Security
Nothing happens automatically. You have to actively enroll through Social Security. Miss your Initial Enrollment Period and you'll typically wait until the next General Enrollment Period (Jan 1 – Mar 31), with coverage starting the month after enrollment.
The penalty that follows you
Without other creditable coverage, the Part B late penalty is 10% added to your premium for every full 12 months you went without it — for life. The Part D penalty is 1% of the national base premium for every month without creditable drug coverage, also for life.
What to verify
If you have any other coverage (employer, VA, TRICARE, retiree), confirm in writing how it counts for each part of Medicare — the rules are different. For Part B, only active employer group coverage based on current employment counts; COBRA and retiree coverage do NOT prevent the Part B late penalty. For Part D, many retiree drug plans ARE considered creditable — but you should never assume. Every plan that offers drug coverage is required to send an annual Creditable Coverage Notice; read it, and keep it.
When in doubt, call Social Security (1-800-772-1213) or talk to a licensed Medicare agent before making a change.