Initial Enrollment Period — Your 7-Month Window
When you turn 65, you have a 7-month Initial Enrollment Period (IEP) to sign up for Medicare. The IEP runs from 3 months before your 65th birthday month through 3 months after. Enrolling in the first 3 months (before your birthday month) ensures coverage starts on your birthday. Enrolling in or after your birthday month delays coverage start by 1–3 months. The IEP covers Part A, Part B, and Part D enrollment. Most people are automatically enrolled in Part A and B at 65 if already receiving Social Security; if not receiving Social Security, you must actively enroll.
Special Enrollment Periods
If you miss your IEP, you can enroll during a Special Enrollment Period when certain life events occur — most commonly: losing employer-sponsored coverage (8-month SEP from the date coverage ends or you stop working, whichever comes first), moving out of your plan's service area, or other qualifying events. Outside of a SEP, Medicare enrollment is restricted to the Annual Enrollment Period (October 15 – December 7) for plan changes, or the General Enrollment Period (January 1 – March 31) for late Part B enrollment (with penalties).
Late Enrollment Penalties
Part B late penalty: 10% of the standard Part B premium added permanently for each full 12-month period you were eligible but didn't enroll. A 2-year delay adds 20% — permanently, for as long as you have Medicare. This is the most significant enrollment timing consequence and affects many people who didn't realize they needed to enroll.
Part D late penalty: 1% of the national base beneficiary premium for each month you went without creditable drug coverage after first eligibility. Also permanent. Avoid by enrolling in Part D even if you take no medications — select the lowest-premium plan in your area.
If You Have Employer Coverage at 65
If you're still working at 65 with employer-sponsored insurance from a current employer with 20+ employees, you can delay Medicare Part B without penalty. Medicare becomes secondary to your active employer coverage. Once you retire or lose that coverage, you have an 8-month SEP to enroll in Part B without penalty. Critical note: COBRA and retiree coverage do NOT count as "active employer coverage" — if your only coverage is COBRA or retiree insurance, enroll in Medicare when you first become eligible to avoid penalties.
How to Enroll — Step by Step
Part A and Part B: enroll online at ssa.gov/medicare, in person at your local Social Security office, or by calling SSA at 1-800-772-1213. Medicare Advantage (Part C) and Part D plans: compare at medicare.gov/plan-compare, then enroll with the plan directly or through your state's SHIP counselor. SHIP (State Health Insurance Assistance Program) provides free, unbiased Medicare counseling — find yours at shiphelp.org. For low-income assistance, see Medicare Savings Programs.
Annual Enrollment Period
October 15 – December 7 is the Annual Enrollment Period when you can: switch from Original Medicare to Medicare Advantage or back, change Medicare Advantage plans, and change or add a Part D drug plan. Changes take effect January 1. Review your plan annually — formularies and networks change, and a plan that was best last year may not be best this year for your specific prescriptions and healthcare needs.
Low-Income Enrollment Help
If you have low income, two programs significantly reduce Medicare costs: Medicare Savings Programs (Medicaid pays Part A and/or Part B premiums and cost-sharing) and Extra Help (reduces Part D costs to near zero). An estimated 1 million eligible people don't receive Extra Help they qualify for. See Medicare Savings Programs and Extra Help for Prescription Costs.