For many years before and after the Affordable Care Act went into effect, many policy-types argued that its success or failure would depend on whether the "young invincibles" would sign up for health care insurance. The argument was that the system needed premiums from young people with few medical expenses in order to balance out the cost of providing care to less healthy people.
The problem with this story is that it is not just young people who have low medical expenses: most older people (pre-Medicare age) also have relatively low health care expenses. In fact, the older healthy people help the finances of the system more since the premiums for the 55 to 64 age bracket average three times the premiums of the youngest age bracket. If we have a healthy young person and a healthy old person, both of whom cost the system nothing, we are much better off if we can get three times the premium from the older person.
The Kaiser Family Foundation did a nice analysis of this issue a few years back showing that even extreme skewing of enrollment by age made relatively little difference to the finances of the system. On the other hand, skewing by health makes an enormous difference.
Vox had a nice piece on the issue of the skewing of the patient pool in the context of President Trump's executive order allowing insurers to offer bare-bones plans that would only be attractive to healthy people. The piece pointed out that Tennessee already has a plan along the lines authorized by Trump and that it has pulled 23,000 people out of the exchanges. It points out that insurance premiums on the Tennessee exchange are among the highest in the country, presumably because more healthy people have been removed from the pool.
However, in making this case, it tells readers:
"Those 23,000 people buying the skimpier health plan are presumably younger and healthier."
While it is very likely that the people buying into the bare-bones plan offered in Tennessee are healthier than the population as a whole, there is no reason to assume they are younger. In fact, the older pre-Medicare age group may actually save more money from this plan than younger people.
So let's leave the age issue out of the discussion, it just generates needless confusion. To keep costs down an insurance pool needs healthy people, it doesn't matter how old they are.