Turning 65 is usually a Medicare Initial Enrollment Period. Marketplace coverage under the is a different system. The two do not stack as a long term pair.
Call your licensed agent a few months before the birthday. Bring the Marketplace application and any employer coverage letter. This page does not enroll you in Medicare.
Missing the Medicare window can mean late enrollment penalties. Dates are federal. Your agent confirms yours.
Your licensed agent can read a deeper version in Field Notes: open the broker article.
FAQ
Short answers you can use on a call with your agent.
Can I keep Marketplace APTC after Medicare starts?
Generally no. Advance premium tax credits are for Marketplace coverage, not for people who have Medicare. Your licensed agent maps the end date.
Devkrest