In a report released online this week by the prestigious Proceedings of the National Academy of Sciences, a team of U.S. and British researchers has found that heavy use of marijuana by teenagers and young adults can reduce their IQ scores by as much as 8 points in later life.

No such results were found in people who smoked the same amount of pot but started using the drug later in life, the report said. People who started young also were found to have weaker memory and attention-focusing skills.

The public policy implications of this finding indicate that, if marijuana use for recreational purposes ever becomes widely permitted, there still remains a strong case for keeping it out of the hands of young people.

That would make it similar to the legal age for buying alcohol and tobacco, set by federal law at 21, which is higher than the age (18) of legal adulthood for other purposes, such as marriage, signing contracts and voting.

The researchers, from Duke University and King’s College of London, analyzed long-term data provided by the University of Otago in New Zealand to evaluate a group of marijuana users of all ages whose intelligence quotients had been measured beginning when they were young and before they had begun using marijuana.

Comparison of IQ scores at age 13 and at age 38 showed a drop only in those who had become “dependent” on pot-smoking prior to turning 18.

The researchers said an average 8-point IQ decrease may not seem like much, but if a person has an average IQ of 100 and drops to 92, it means that instead of measuring a higher intelligence than 50 percent of the general population, that person will only measure higher than 29 percent of it.

Marijuana is reported to be the most widely used illegal drug in the world, with the United Nations estimating that in 2010 there were between 119 million and 224 million regular users between the ages of 15 and 64.

In June, the U.S. government said that 23 percent of high school students reported smoking pot “recently,” more than reported smoking tobacco in the same period, the first year ever in which that was the case.

Marijuana is widely believed to be less harmful than tobacco, but ounce for ounce it has long been known to be more carcinogenic.

Now, it is also been found to be harmful to developing brain matter. Young people should think hard about becoming dependent on it — before it becomes hard for them to think at all.