Well, that's the point about single-payer healthcare. You pay into the pool when you're young and healthy, and you take out from the pool later. If you expect to take out from the pool only when you need it without paying in in advance, the system collapses. Because what you need is more than what you reasonably can be expected to pay in. Unless you're filthy rich.
Besides, young people also get sick. Granted, not as often as the old 'uns, but if they do and don't have insurance, they're majorly screwed. You're gonna die? Too bad you don't have insurance.
Besides, all numbers show that the total economic burden per health benefit is lower in single-payer systems.
No one needs insurance. . .until they do and then, they really really need it.
People get into car accidents, get cancer, young people get diagnosed with Multiple Sclerosis.