Anyone at any time can get sick, get hit by a car, etc. That's why everyone needs it.
Technically, the utility of health insurance is greatest when you are youngest and least when you are oldest. Saving an 8-year-olds life could mean allowing them to live to 80. A 72-year-old that can't get medical care will have at least lived to 72.
The U.S. health care system has it backwards. We should at least be giving heath insurance to the young. You could argue that it is moral to provide for health insurance for retirees, even if they spend way more on health insurance than everyone else and won't get much additional utility from it, but the idea that you would only give guaranteed health insurance to the oldest (and wealthiest age bracket) makes no sense.