Austin Frakt provides one answer:
[T]here is evidence that Medicare improves health outcomes (no surprise there), but also that previously uninsured individuals who become beneficiaries of that program require a lot more care (not such a shock either). They’re less healthy than they would have been had they had insurance before turning 65.
Thus, another way to interpret poor post-65 U.S. mortality relative to peer nations is that we do a worse job of providing access to coverage and the care it facilitates before individuals become Medicare eligible. In short, even mortality for Medicare beneficiaries could be related to the health insurance system. This is not really surprising since many causes of death are related to life choices made and health care received over many years. It is not inconceivable that better care at an early age can prolong life even for those who make it to 65.