It depends on the size of the employer and your specific circumstances. It's essential to compare the costs and benefits of Medicare to your employer's plan. If your employer has less than 20 employees, then you are required to enroll into Medicare A & B when you first turn 65. You do not need to purchase a Supplement Plan or Advantage Plan if you choose to keep your employer’s coverage, but you do need Parts A & B or you will face a late enrollment penalty.