Who MUST Enroll in Medicare at Age 65?

I have a prospect in Colorado turning 65 in June 2016. He and his wife turning 65 in August 2016 are employee and spouse on their 4 employee group plan. MUST they enroll in Medicare because group is less than 20 employees? And, do the employee and spouse rates jump upon turning 65? If they MUST enroll in M/C A and B is that a federal requirement or a requirement imposed by the carrier and actually or usually in the group contract? Thanks for any guidance given.

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