Can an employer of less than 20 employees, and which offers group health coverage that coordinates with Medicare (i.e., pays only what Medicare does not pay, for Medicare-entitled individuals) simply terminate group health coverage of active employees and their dependents who reach age 65?
The small employer exemption from the Medicare Secondary Payer Act says that Medicare can be primary payer for eligible employees, but I don't think it also creates an exemption from the ADEA as far as who gets offered benefits. . . .

You are right. The MSPR only provide an exception to the normal rule that Medicare is secondary to the employer's group health plan. The MSPR do not give the employer the right to violate any other statute like the ADEA.