The health insurance sign-up data released Wednesday afternoon by the Obama administration tell us a lot of things.They tell us how many people have selected an insurance plan through the new health care exchanges (106,185 Americans) and where they came from (79,391 in state marketplaces and 26,794 in the federally-run exchanges).

But there's one key number that's missing, a figure we absolutely need to understand whether the health care law can succeed. And that's the ratio of young enrollees to  older enrollees.

As Ezra Klein and I wrote earlier this spring, the White House is bent on getting young people signed up on the health care exchanges. States are in the same boat: This is why you see ads from groups like this Colorado nonprofit that are aggressively microtargetting what might best be described as the "bro demographic."

There's a good reason for this heightened focus on wooing younger health insurance customers: Holding premiums down depends on getting the "young invincibles" into the insurance pool. They tend to have lower health care costs than people who are their parents' age, meaning they won't likely be asking the insurer to pay as many medical claims.

Typically, younger people contribute more to the insurance plan through premiums than they draw out in claims, unless, of course, they suffer a catastrophic health care issue. That means they help keep premium costs low, making the insurance products in the new marketplace more attractive to all shoppers.

That's a key metric for judging whether the health care law is successful: Is it expanding access to affordable health insurance coverage? The other important metric, of course, is how many people are purchasing coverage.

The White House has estimated that it needs about 2.7 million of the expected 7 million enrollees this year to be under age 35. What's important here isn't necessarily the numbers but the ratio -- that for every three older people who sign up, the administration believes it needs to persuade two young people to obtain coverage.

Right now, we don't know how close the White House is getting to that ratio.

"We have not yet collected data from states on demographics," Chiquita Brooks-LaSure, policy director at the U.S. Health and Human Services Department's Center for Consumer Information and Insurance Oversight, told reporters Wednesday. "On the federally facilitated marketplace that is something that is a priority, but we first wanted to get out the enrollment number."

She did not give a timeline for when that data would be released.

In the first month of enrollment, more older people than young would be expected to sign up, as the older folks would be the most anxious to get coverage because they would anticipate having higher health care costs. But any data on the age breakdown of the health exchanges' enrollees are going to be very important to understanding whether or not Obamacare will work.