The Affordable Care Act’s new insurance exchanges are designed to offer health plans at rates close to those currently only available to employees of large companies. But even young people who are offered health insurance through their workplace often turn it down, according to a new study.

Less than 64% of employees under 30 working for large companies sign up for health insurance when it is offered by their employer, compared with at least 76% of all employees over 30, according to a new report by benefits administrator ADP. And Americans ages 18 to 34 are the least likely of any age group to be insured: About a third lack coverage, according to the Centers for Disease Control and Prevention.

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Policy analysts worry that the relatively low numbers of young people who buy health insurance at their job could indicate that few will buy it when the insurance mandate kicks in next year. Getting more so-called millennials, roughly those between the ages of 18 and 34, to enroll in health plans in the new insurance exchanges is important to the success of the Affordable Care Act, as the healthy young people tend to pay more into insurance plans than they use, buoying plans sunken by the expenses of sicker workers.

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Indeed, if all eligible workers enrolled in their workplace health plans, each employee could expect to pay 14% less in premiums because the young peoples’ contributions would balance the health expenses of their older colleagues, says ADP vice president Christopher Ryan.“If you’re an employer, in the long run, to keep premiums down, you want to have the young people as well as the older people participating,” he says.

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As much as 70% of a company’s workforce doesn’t use any health coverage at all in a year, Ryan says, but insurance plans still need their contributions to pay for those who use more.

The health reform law offers several new incentives intended to lure young adults to insurance plans, say experts, including the ability to stay on a parents’ plan until age 26, cheaper individual insurance options exclusively for people under 30, and mandated free birth control pills for those with insurance.

But young people may be even less likely to buy health insurance on the new exchanges created by the Affordable Care Act than they are to buy it at the workplace, because large employers’ plans are generally a better deal than what the exchanges will offer, benefits experts say. While the exchanges are intended to bring the cost of individual health insurance down to cheaper group rates that are more in line with large employers’ prices, that coverage may not be as robust as what’s offered in typical employer plans. And some younger workers with individual plans may see their premiums go up as much as 29%, according to America’s Health Insurance Plans.

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Policy analysts have said that the reason so few 20-somethings typically have coverage is because they’re less likely to have income or jobs with benefits—something the new insurance exchanges, opening Oct. 1, are designed to address by offering health plans to the unemployed and tax credits for low earners. Indeed, fewer young workers have been allowed to enroll in their employer’s health plan in recent years, likely a consequence of companies hiring more freelancers and less full-time staff, Ryan says: The number of under-30 employees eligible to join their company’s health plan declined 5% between 2010 and 2013, a sharper drop than 30-plus workers experienced.

But even though a decline in access to health benefits makes employer insurance a rarer privilege among young workers, fewer sign up even when it is available, compared with their older colleagues. Among employees in their 30s, for example, 2% more took insurance when employers offered it between 2010 and 2013, while hardly more 20-somethings signed up than did so three years earlier.

What else might be behind the low proportion of young adults with insurance? For one thing, people under age 26 may be less interested in their company’s insurance because they can stay on their parents’ health plan, thanks to the Affordable Care Act, Ryan says. And even though employer plans are typically cheaper than individual insurance, young workers, who typically have lower salaries, may still find them too expensive: While the penalty for not having insurance will be 1% of income in 2014, for someone earning roughly $45,000, employer health plan premiums would eat up 3% to 5%, Ryan says. A different recent study by ADP found that when people make at least $45,000, more than 80% of them will enroll in health insurance, but only 58% of people who make between $20,000 and $25,000 will buy coverage. At incomes of $15,000 to $20,000, the rate drops to just 37%.

But there is another reason younger people may also not see as much value in employer health benefits as older individuals, some social scientists say: Without children or spouses, they might be more willing to gamble on their continued health.

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