New Obamacare numbers -- how many are the young ones needed to make it work?

http://www.pennlive.com/news/2016/03/new_obamacare_numbers_--_how_m.html

About 12.7 million people are enrolled in health plans sold through an electronic exchange that's part of the Affordable Care Act, or Obamacare, the federal government said Tuesday.
That's about 1 million more that the number enrolled at this point last year.
But only about 28 percent of enrollees, or 3.5 million, are between the ages of 18 and 34 -- the younger, healthier people needed to offset the costs of older, sicker ones. The government has said it needs slightly more than a third of people to be in that age range.