Not true.
As long as one is employed the age does not matter until age 70-1/2.
I just had a workmate retire at 68 and he was not obligated to sign up for part A of Social Security until his work related full medical coverage was no longer in force.
OK. But it is still mandatory to participate when you start collecting SS or reach 65 - whichever comes later. This makes it a universal, mandatory health program - which the government completely controls. Even if an individual retains another health plan, Medicare is still the first payer and therefore controls the price, treatment, and rules.