If you are actively working and covered by an employer's health plan (with 20 or more employees), you do not strictly have to enroll in Part A right at 65. However, since it is premium-free for most, many choose to enroll anyway to serve as secondary hospital insurance. Importantly, if you have a High Deductible Health Plan and want to continue contributing to a Health Savings Account (HSA), you must delay enrolling in Part A, as any Medicare enrollment disqualifies you from making new HSA contributions.