Which of the following statements regarding Medicare and employer group health plans is correct?

The statement that group coverage can supplement Medicare is accurate because many individuals choose to maintain their employer-sponsored health plans even after they become eligible for Medicare. In such cases, the employer's group health coverage can work alongside Medicare to help cover costs not fully addressed by Medicare. This provision allows for more comprehensive coverage by filling in the gaps left by Medicare, which typically includes premiums, copayments, deductibles, and services that may not be covered at all.

As for the other statements, Medicare is not necessarily primary for all employees; it depends on various factors such as the size of the employer and the employee's age. Additionally, coverage does not simply stop at age 65, as many employer plans continue indefinitely regardless of the employee's age. Lastly, Medicare's status as primary or secondary insurance is based on specific rules influenced by the size of the employer, generally being secondary if there are more than 20 employees and primary for those with fewer employees. Therefore, the role of group health plans as a supplement to Medicare provides important context and value for those eligible for both types of coverage.