Article originally posted by CNBC

Written by Sarah O’Brien

If you get your health insurance through the public marketplace and are nearing age 65, don’t forget about Medicare.

The general rule is that you must enroll when you reach that age unless you have qualifying coverage elsewhere — and plans through the exchanges (whether federal or state) do not count.

“You need to be prepared to make that change,” said Karen Pollitz, a senior fellow with the Kaiser Family Foundation. “Otherwise you can face [costs] for being late to enroll in Medicare and for being late getting out of the marketplace.”

Of the 12 million or so people who have health insurance coverage through the marketplace, roughly 3.4 million are ages 55 to 64 — meaning some of them are approaching Medicare eligibility and will need to sign up.

“If you were getting a subsidy on the marketplace plan … Uncle Sam can bill you for all the subsidy dollars you’ve received since turning 65 and not leaving that plan,” said Danielle Roberts, co-founder of insurance firm Boomer Benefits.

Individuals who are already receiving Social Security payments — i.e., they started those benefits before their full retirement age as defined by the government — generally will be automatically enrolled in Medicare but will still need to cancel their coverage through the marketplace, Pollitz said.

Your initial Medicare enrollment period starts three months before the month of your 65th birthday and ends three months after it (seven months total).

While most people pay no premium for Part A (hospital coverage), that’s not the case for Part B (outpatient care). For 2021, that standard amount is $148.50 monthly, although higher earners pay more.

In addition to Parts A and B — also called original Medicare — there is Part D, which provides prescription drug coverage. It, too, comes with extra monthly charges for beneficiaries with higher income (see charts).

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