Medicare is a wonderful program for seniors. We have many older than 55 who have been laid off and are no longer eligible for health insurance through any employer. Also those who are self-employed cannot receive employer-based health insurance.
Instead, allow anyone 55 and older to buy into Medicare at the price of 10% above the actuarial calculated cost of providing care and administration. Use that 10% profit to help cover the cost of those on Medicare (who are over 65).
This program would be entirely voluntary. It would protect Medicare, and allow those 55 and over to get insurance that is affordable. In many cases, those older 55 can't get insurance on the private market.