most of those without health insurance are young and do not have major medical expenses.
Well, I suppose that the uninsured could be young people with no major medical expenses but nonetheless they are less healthy than other young people.
But honestly, I think the discrepancy between these two statements is genuine. I don't think we know very much about the nature of the uninsured population, and the guesses tend to be all over the map. As Greg Mankiw once pointed out, many uninsured are eligible for Medicaid but have not enrolled. Some others have declined employer-provided health insurance.
I don't claim to have a handle on the uninsured population.