When you turn 65 and DO NOT have group health insurance, Medicare will be your primary health insurance. You can enroll 3 months prior, and no later than 3 months after your 65th birthday.
If you miss this window you can be penalized by Social Security and may be ineligible for Medicare Supplement Plans.
This guide is meant to be for learning purposes only. There can be unique situations that need to be considered when going on Medicare.