Adolescents who smoke marijuana as early as 14 do worse by 20 on some cognitive tests and drop out of school at a higher rate than non-smokers. But if they hold off until age 17, they're less at risk, and performed equally well as adolescents who did not use cannabis.

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Adolescents who smoke marijuana as early as 14 do worse by 20 on some cognitive tests and drop out of school at a higher rate than non-smokers. But if they hold off until age 17, they're less at risk, and performed equally well as adolescents who did not use cannabis.