As of the end of December, 2.1 million Americans had signed up for actual health insurance plans on Obamacare. (All the others were eligible for Medicaid.) Here is the breakdown of age groups purchasing insurance plans:

Obamacare is not sustainable if over half of health insurance purchasers are over 45. The crucial 18-34 age group (young adults in good health) is 24%, not nearly enough to keep the system going.

In addition, the majority of insurance purchasers on the Obamacare exchange are getting subsidies:

This means that older, sicker, poorer people are flocking to the exchange, while those who are younger, healthier, and (relatively) better off are staying off the exchange.

And remember, only 2.1 million out of 3.7 million (57%) on the exchange are actually buying insurance.
1.6 million (43%) were signed up for Medicaid. Perhaps this explains the coming bailout:

Bailing Out Health Insurers and Helping Obamacare

Robert Laszewski—a prominent consultant to health insurance companies—recently wrote in a remarkably candid blog post that, while Obamacare is almost certain to cause insurance costs to skyrocket even higher than it already has, “insurers won’t be losing a lot of sleep over it.” How can this be? Because insurance companies won’t bear the cost of their own losses—at least not more than about a quarter of them. The other three-quarters will be borne by American taxpayers.

For some reason, President Obama hasn’t talked about this particular feature of his signature legislation. Indeed, it’s bad enough that Obamacare is projected by the Congressional Budget Office to funnel $1,071,000,000,000.00 (that’s $1.071 trillion) over the next decade (2014 to 2023) from American taxpayers, through Washington, to health insurance companies. It’s even worse that Obamacare is trying to coerce Americans into buying those same insurers’ product (although there are escape routes). It’s almost unbelievable that it will also subsidize those same insurers’ losses.