Same goes for health insurance... if you want to own health insurance. And as we know a 20 year old does not feel they need health insurance as an example.
I really hate to keep correcting you but in every state that I know off automobile insurance is a requirement if you want to own a car. That is only one of many things that governmental authorities can and do require people to comply with.
That said, the way I understand it, people who opted out would still have the same private coverage, or they decided (at that point, it would be a decision) that they did not need health care, and the two groups would be treated the same as they are now. 