As enrollment statistics in the new health insurance marketplaces start to become available, there is a growing focus on whether the enrollment of so-called “young invincibles” will be sufficient to keep insurance markets stable. Enrollment of young adults is important, but not as important as conventional wisdom suggests since premiums are still permitted to vary substantially by age. Because of this, a premium “death spiral” is highly unlikely. Why does the age distribution of enrollees matter? The Affordable Care Act (ACA) requires insurers in the individual market to cover anyone who wishes to enroll and restricts how . . .