Is there a difference in mental functioning for chronic users of marijuana who start at an earlier age than those that get a later start? Researchers in Brazil tested those that started to use marijuana before the age of 15 and compared them to chronic users who started after 15.
Early starters performed significantly poorer than late starters in categories such as sustained attention, impulse control, working memory, problem solving, cognitive flexibility, and planning.
Researchers drew the conclusion that starting chronic use of marijuana at an age when the brain is still developing may have a greater negative effect on cognitive functioning.
Abstract: Cannabis use before age 15 and subsequent executive functioning, The British Journal of Psychiatry