The insurance exchanges created by the Affordable Care Act (ACA) are not intended for Medicare beneficiaries to purchase health insurance. However, because Medicare does not include a dental benefit, Medicare beneficiaries may be able to purchase dental plans on the exchanges in certain states.
Those looking to purchase dental benefits should be aware that some plans include coverage for children, which increases premiums. Also, in the 36 states in which the federal government runs exchanges, only individuals who are purchasing health insurance may purchase dental coverage.
Check out this New York Times post for more information.