Children and teenagers are at risk of developing mental illness at younger ages than psychiatrists traditionally believed, a new study from the National Institute of Mental Health has found.

The new report, says NIMH director Lewis L. Judd, "shows that young people under age 20 are in the peak age range for development of depression, obsessive-compulsive disorder, phobias and substance abuse disorders."

The study was based on data collected by NIMH researchers in interviews with 18,000 people in five U.S. cities conducted between 1980 and 1984. Known as the Epidemiologic Catchment Area Program, it is the largest population survey of mental illness ever conducted.

Results were announced at a hearing in Los Angeles, the first of a series of hearings on child and teen mental health problems.

Researchers compared findings from the interviews with the generally accepted ages of onset listed for these mental disorders in the most recent edition of the widely used manual of psychiatric disorders. They found statistically significant differences. For example, the manual -- "DSM III-R" -- suggests that the age of onset of alcohol abuse is "usually in the late twenties, thirties and forties."

The NIMH report finds "the hazard rates are highest between the ages of 15 and 19 years." That is also the peak period for depression, some panic disorders, obsessive compulsive disorder and drug abuse.

Frederick Goodwin, chief of the Alcohol, Drug Abuse and Mental Health Administration, of which NIMH is a part, said that because of the new study "we know that our prevention, detection and treatment activities must begin earlier, too."

Because depression is believed to be a major factor in many of the suicides of about 2,000 teenagers each year, the NIMH team urges schools and families to be especially alert for such signs of depression as loss of pleasure in usual activities, loss of energy, feelings of sadness, sudden problems at school and worrisome changes in behavior.

Most of these disorders can be successfully treated but, Judd said, only about one in every five of the 7.5 million Americans under age 20 estimated to have mental disorders is getting help.