Early exposure to marijuana most damaging.

In Brain Circuitry, Brain Development, Brain General Structure and Function, Brain Science, Memory and Learning by Brainy Days Ahead

A new study by researchers at the Université de Montréal found that adolescents who smoke pot as early as 14 do worse by 20 on some cognitive tests and drop out of school at a higher rate than non-smokers. They also concluded that, although the more teenagers delay smoking marijuana until they’re older, the better it is for their brains, there may be less ill effect if they start after age 17.