Because most of your medical care will be paid for by Medicare.
There is no legal requirement to sign up for Medicare. However, some private insurance plans impose penalties on Medicare-eligible persons who do not sign up.
Although this answer varies with the individual's health and financial resources, the answer is almost always, "yes." And, if you have private health insurance, your private carrier will almost surely require you to take Medicare Part B (SMIB).
As far as I know, Medicare does not require you to sign up. However, your group health insurance will almost certainly require you to do so.
Medicare, Medicaid and Private Insurance
Yes, because there will probably be a penalty for late enrollment, and your employer's health insurance will probably require you to enroll in Medicare.
I'm not familiar with UMWA health insurance, but most private carriers expect providers to bill Medicare first.
Members of Congress pay FICA/Medicare. They also pay health insurance premiums at rates comparable to the private sector.
Private health insurance companies generally avoid the elderly. If an elderly person currently does not have medical insurance, their best bet is to find a job that will grant group insurance regardless of their age, or to apply for Medicare.
No, you can keep your insurance.
Medicare does not require anyone to register at age 65 or any other time. However, your health insurance company might require you to register in order for Medicare to be your primary insurance.
The current regulations require tat you have some sort of minimal health insurance, either through an employer, purchased on your own, or from the government (medicaid) if you qualify based on income and resources. If you already have health insurance from an employer or otherwise that is comparable to or better than medicare than you can keep it and do not need to apply for medicare.
You do not determine which of your insurance policies are primary in cases where you have multiple health insurance policies. The Federal government passed a law several years ago making Medicare secondary to any other health insurance that you have through an employer or retirement program. This transferred billions of dollars per year from the Medicare and Medicaid programs to private insurance companies all at one time.