by Cara Buckley

It is one of the biggest black holes in the nation’s health care system: vast numbers of young adults between the ages of 19 and 29 with no health insurance.

To plug the gap, Gov. David A. Paterson is preparing a plan that will allow many more young adults to be claimed as dependents on their parents’ health insurance plans, a move he described as a significant step toward achieving universal health care in New York.

The plan was praised by some health care experts as a major step forward, but early evidence from the roughly two dozen other states that have adopted similar programs suggests that their effectiveness in shrinking the ranks of the uninsured has been modest at best.

“We’ve reviewed the laws and regulations from all 25 states,” said Joel C. Cantor, the director of the Center for State Healthy Policy at Rutgers University, who is studying the national impact of the state laws on insuring young adults. “This is about as incremental as incremental gets.”

Under Mr. Paterson’s proposal, employees would be able to claim dependents up to age 29 as part of their health care coverage, up from the current limit of 19, or 22 for full-time students.

Young adults represent one of the largest groups in the country lacking insurance. According to the Commonwealth Fund, a nonprofit research group in New York, 13.7 million people between the ages of 19 and 29 across the country went without medical insurance in 2006, a number that represented nearly a third of the country’s uninsured population that was not elderly.

In New York, 800,000 people in that age group have no insurance, according to the governor’s office, or about 31 percent of the state’s uninsured population. About 80,000 young people could be insured within the first year of the governor’s plan, which is subject to approval by the Legislature. The estimate is based on enrollment rates in New Jersey, where a similar law was adopted in 2006.

It is unclear how much support the proposal will have in the Legislature, but Mr. Paterson said neither businesses nor the state would shoulder the extra cost, which instead would be absorbed by the families who opted for expanded coverage.

“The plan seems like a very legitimate step in the right direction toward improving access to health care,” said Austin Shafran, a spokesman for the new Senate majority leader, Malcolm A. Smith. “We’re really taking a look at it.”

There are numerous reasons young adults tend to be uninsured. Public programs and parents’ policies often set a cutoff for dependents at age 19 or at graduation from college. Young people often start their careers working part time or for employers who do not offer health coverage. Or they decide not to buy insurance because of the cost, or simply because they believe their youth makes them invincible.

In that age group, members of ethnic and racial minorities are more likely to be uninsured than whites — more than a third of African-Americans and half of Hispanics nationwide are uninsured, compared with roughly one in four whites.

The raising of the age limit mandated by numerous state laws was intended to bridge that age gap, at least for dependents whose parents have coverage. But gauging its impact has been difficult.

Few states where the age gap now is higher have gathered data about how many young people obtain insurance. One exception is New Jersey, which allowed parents to claim adult children up to the age of 30 (the age was recently raised to 31) as long as their dependents were unmarried, had no children of their own and were living in the state or enrolled full time in college.

About 15,000 young adults have been enrolled as dependents since the state increased the age limit in 2006, with 5,000 using it for a short time before dropping out and 10,000 still enrolled, said Marshall McKnight, a spokesman for New Jersey’s Department of Banking and Insurance.

“It is considered a success intended to provide temporary coverage for those who needed it,” Mr. McKnight said.

But Wardell Sanders, president of the New Jersey Association of Health Plans, said the impact of the age-limit extension had not been as great as state officials had envisioned because of the law’s restrictions and because federal laws that limited the state program.

“I think the folks in New Jersey may have overestimated the number of folks potentially eligible,” Mr. Sanders said.

While states can regulate insurance companies, the federal government regulates employers that self-insure. Generally, larger employers with 500 workers or more self-insure, meaning they bear the full financial risk of the costs, while small and midsize employers buy insurance from companies that bear the risk. As a result, Mr. Cantor said, 55 percent of the nation’s workers who have insurance under an employer’s plan are unaffected by state regulations that increase the maximum allowable age to claim dependents because the employer self-insures.

New York’s law would be structured along the same lines as New Jersey’s, but its restrictions would be more flexible, according to the governor’s office. Adult children could be claimed as dependents even if they have children or are enrolled in college part time or live in a different state as long as their insurance policies are subject to New York law.

The law, like those in other states, would not cover young people whose parents have no insurance, or are unemployed, a group that has been growing since the economy’s slowdown. Still, policy experts said the proposed law was an improvement.

“Moving the age up to 29 is very significant for lot of young adults who don’t have coverage under an employer,” said Sara R. Collins, an assistant vice president at the Commonwealth Fund. “It definitely addresses some of the problem that dependents face when they graduate from high school and don’t go to college and whose parents are affected by the laws.”

Utah was the first state, in 1994, to increase the age limit for dependents. Most states set the maximum age at which parents can claim their children at 25. The laws vary by state, but most require that adult children be unmarried and either be full-time college students or live in the same state as their parents — sometimes at the same address.

“It’s the simplest solution, but not an easy solution in any stretch,” said Kim Holland, Oklahoma’s insurance commissioner. “And we’re not sure whether it’s the best solution or not.”