Do I have to enrol in Medicare at 65 if I have an HSA?

If you are still working at 65 with a large employer's plan, you can usually delay Medicare without penalty and keep contributing to your HSA. The trap is the backdating rule: when you eventually enrol, Part A can be made effective up to six months retrospectively, and any contribution made during those retroactive months becomes an excess contribution.

The practical fix is to stop HSA contributions six months before you intend to enrol or claim Social Security, since claiming Social Security automatically enrols you in Part A. Spending from the account is unaffected — you can use the balance for Medicare premiums and medical costs for as long as it lasts.

An estimate for planning, not a quote. Medicare and the Marketplace set your real figures.