NEW YORK — Teens who routinely smoke marijuana risk a long-term drop in their IQ, a new study suggests.

The researchers didn’t find the same IQ dip for people who became frequent users of marijuana after 18. Although experts said the new findings are not definitive, they do fit in with earlier signs that the drug is especially harmful to the developing brain.

‘‘Parents should understand that their adolescents are particularly vulnerable,’’ said lead researcher Madeline Meier of Duke University. Meier and colleagues reported their work online Monday in the Proceedings of the National Academy of Sciences.

Study participants from New Zealand were tested for IQ at age 13, presumably before any significant marijuana use, and again at age 38.

The mental decline between those two ages was seen only in those who started regularly smoking marijuana before age 18.

‘For some it’s a legal issue. But for me it’s a health issue.’

Richie Poulton, a study co-author and professor at the University of Otago in New Zealand, said the message of the research is to stay away from marijuana until adulthood if possible. ‘‘For some it’s a legal issue,’’ he said, ‘‘but for me it’s a health issue.’’

Marijuana is the most popular illegal drug in the world, with somewhere between 119 million and 224 million users between the ages of 15 and 64 as of 2010, the United Nations reported. Within the United States, 23 percent of high school students said they had recently smoked marijuana, making it more popular than cigarettes, the federal government reported in June.

Young people ‘‘don’t think it’s risky,’’ said Staci Gruber, a researcher at the Harvard-affiliated MacLean Hospital in Belmont, Mass. Gruber, who did not participate in the new work, said the idea that marijuana harms the adolescent brain is ‘‘something we believe is very likely,’’ and the new finding of IQ declines warrants further investigation.

Scientists said the new research is an advance because its methods avoid criticisms of some earlier work, which generally did not measure mental performance before marijuana use began.

‘‘I think this is the cleanest study I’ve ever read’’ that looks for long-term harm from marijuana use, said Dr. Nora Volkow, director of the National Institute on Drug Abuse, which helped fund the research.

The study drew on survey data from more than 1,000 people in New Zealand, everybody born in the town of Dunedin during a year-long span ending in 1973.

In addition to IQ tests, they were interviewed five times between ages 18 and 38, including questions related to their marijuana use.