The 15 percent of U.S. high school seniors who abuse alcohol and drugs first began using them at age 14 or 15, according to a study that’s one of the first to track substance-abuse trajectories during this key period.

The finding, reported today in the Archives of General Psychiatry, suggests that prevention programs should begin early in adolescence before problems arise, rather than when abuse becomes obvious, said Joel Swendsen, the lead author.

Teenagers whose substance abuse caused problems socially, in school or with the law, started drinking or doing drugs when they were 14 or 15 years old, the research found. By the time kids reach age 18, almost half consumed at least 12 drinks a year and 15 percent were abusers, according to the report. About 43 percent of older teens used drugs at least once and 16 percent abused them, the study found.

“This is showing us that we should intervene well before 14 or 15 years old,” said Swendsen, director of research at the National Center for Scientific Research at the University of Bordeaux, France, by telephone. “The earlier you can intervene, the bigger the payoff in terms of preventing these problems.”

The researchers re-analyzed data from a national face-to- face survey of 10,123 adolescents ages 13 to 18 years in the U.S., conducted between February 2001 and January 2004. Previous reports from the earlier research only looked back a year on student drug use, according to the researchers from the National Institutes of Health in Bethesda, Maryland.

The most-recent report found that white and Hispanic kids were more likely than those of other racial groups to consume alcohol or drugs. Boys were more likely than girls by the age of 17 or 18 to drink or do drugs, the study showed.

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Nicole Ostrow, ©2012 Bloomberg News