Group 1 - Turning 65 is a significant milestone as it marks eligibility for Medicare health coverage, but individuals still working may not need it [1][3] - Those with employer-provided health insurance can sign up for Medicare as secondary insurance, but may choose to delay enrollment due to costs associated with Medicare Part B [1][4] - Individuals who delay signing up for Medicare risk incurring lifelong surcharges on their Part B premiums unless they qualify for a special enrollment period [3][8] Group 2 - The initial Medicare enrollment window spans seven months, starting three months before the 65th birthday and ending three months after [4] - If still employed and covered by a qualifying group health plan, individuals can delay Medicare Part B enrollment without penalties [4][5] - Upon leaving a job or losing health coverage, individuals qualify for a special enrollment period lasting eight months to sign up for Medicare [5] Group 3 - Qualifying group coverage typically applies to health insurance plans with 20 or more employees, while certain plans like COBRA and retiree insurance do not count as qualifying [5][7] - Failing to sign up for Medicare on time results in a 10% surcharge on Part B premiums for each 12-month period of eligibility without enrollment, which remains for life [8]