Why is the participation of young invincibles important to the success of the aca health insurance marketplaces?

First of all, young invincibles are young people, usually between 18 and 34 years old, that expect to remain healthy and therefore are not willing to buy health insurance.

The problem is that effectively young invincibles use less health care than other age groups but they serve as a balance for people with preexisting conditions that use health care a lot.

When the amount of young invincibles that acquire insurance is too small compared to preexisting conditions groups, health care costs will increase due to adverse selection and health care premiums will also increase.