ABC News: Chronic pot smokers beware. A new study found that regularly smoking marijuana may lower cognitive function, especially if the person starts smoking before 16 years of age. Study authors from Harvard-affiliated McLean Hospital wanted to examine the effects of chronic marijuana use on brain function. In a paper presented today at the annual meeting of the Society for Neuroscience in San Diego, Dr. Staci Gruber, lead author of the small study, reported that study participants who began smoking pot before 16 years of age performed significantly worse on cognitive function tests than both non-smokers and those who became chronic smokers later in life.