If your employer has over 20 employees and therefore the employer plan would be primary, no they cannot per the Social Security Act. However, if the employer has less than 20 employees, employers are not required to cover you any longer because their plan would be primary and the cost of continuing to provide insurance coverage can be a significant expense to the employer.
Remember, you can enroll onto Medicare (via Social Security) in the 3 months prior to your birthday month, the month of your birthday, or within the 3 months following your birthday month. If you miss this window and did not have employer coverage, you will incur a penalty for delaying in signing up for Part B. Part of that delay will result in your not being able to ask for enrollment in Part B until Jan-March of the following year, resulting in an effective date of 7/1 and a 10% penalty that you will incur for your lifetime. Also be aware that without Medicare Part A and Part B, you will not be eligible to enroll onto a Medicare Advantage (HMO) plan or a Medicare supplement plan.