WASHINGTON - The U.S. mental health system has huge gaps that prevent millions of people with psychological problems, including children and teens, from receiving effective treatment that could prevent tragic consequences, experts told U.S. lawmakers on Thursday.

Just over a month after the shooting rampage in Newtown, Connecticut, experts told a Senate hearing that three-quarters of mental illnesses emerge by age 24, but fewer than one in five youths with diagnosable problems receive treatment that could avoid later problems including violence and suicide.

Overall, experts said as many of 45 million Americans experience mental illnesses such as depression, eating disorders, post-traumatic stress disorder and drug abuse each year. But only 38 percent get treatment.

"These are the chronic disorders of young people," said Dr. Thomas Insel, director of the National Institute of Mental Health.

The hearing, before the Senate Health, Education, Labor and Pensions Committee, was held in response to the shootings at Newtown's Sandy Hook Elementary School, where a young 20-year-old man described as having mental issues gunned down 26 people including 20 young children with assault rifle on December 14. It was the first time the committee has addressed the issue of mental health since 2007.

The Newtown tragedy and other mass shootings in recent years have ignited a debate about gun control and mental health, including a push by President Barack Obama for stronger gun controls and better mental health training for schools and communities.

But the committee's Democratic chairman, Tom Harkin, warned against drawing a bold parallel between mental illness and violence against others.

"One of the most insidious stereotypes about people with mental illness is that they are inherently violent," said the Iowa senator. "People with mental illness are much more likely to be the victims of violent crimes than they are to be perpetrators of acts of violence."

Insel said a relatively small number of mentally ill people, who suffer from symptoms such as paranoia and hallucinations, are violent. "Far more common than homicide is violence against the self," he said, pointing out that 90 percent of the 38,000 suicides each year involved mentally ill people.

All told, he said, the risk of violence, including suicide, among people who develop mental illness is 15 times greater without treatment.

Experts cautioned that treatment should avoid powerful drugs for children who are often vulnerable to side-effects and recommended extra care to ensure that the normal behavioral problems of childhood and adolescence not be mislabeled as mental illness.

In response to the Newtown tragedy, Obama has announced a series of initiatives intended to help teachers and other adults identify children, adolescents and young adults with mental illness and ensure they receive treatment.

Experts said Obama's healthcare reform law is expected to lead to the biggest increase in mental health access in a generation. After January 1, 2014, it is scheduled to extend health coverage to millions of Americans currently locked out of the $2.8 trillion U.S. healthcare system because of a lack of insurance.

Pamela Hyde, administrator of the U.S. Substance Abuse and Mental Health Services Administration, said that as many of 10 million people with mental illnesses could gain access to care as a result of the Patient Protection and Affordable Care Act. "Prevention works. Treatment is effective. And people recover," she said.

But Senator Patty Murray, a Democrat from Washington state, expressed concern that the Obama administration is not moving fast enough to produce detailed rules on how mental health access should be made available through new state-based online health insurance marketplaces being set up under the law.

"It's really essential that we see a final rule before April," Murray told Hyde. "Our states are working on the exchanges and they need that clarity. I can't urge you strongly enough."

Hyde said a final rule on essential benefits is due next month. But she could not say whether a separate rule on mental health parity would meet Murray's deadline.