Study participants from New Zealand were tested for IQ at age 13, likely 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 pot before age 18.This effect is real. But it isn't restricted to marijuana. Between the ages of about 16-22 the brain goes through a process called the "Great Pruning". After a childhood of forming connections, it goes through and cleans out the ones that aren't being used. It sort of 'defrags' your hard drive...
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.”
If you are not actively using your brain, it will get pruned more and you will have less to work with in the future. This is true for all kinds of behavioral restrictions- if you are sick, or incarcerated, or watch TV all the time... whatever. Pot is bad because it facilitates all of those passive behaviors.
Young adults need to be active- studying, working, traveling, so their brains will have a good foundation for the future.
The cumulative effects could be confounded by any number of things. Like sugar metabolism. That affects cognition and wanes as we age too.
