http://www.theglobeandmail.com/life/health-and-fitness/health/your-kids-brain-on-pot-the-real-effects-of-marijuana-on-teens/article21127612/

The female brain matures at age 21, the male brain at age 25. If you do drugs or drinking before your brain matures, you will pay the price in both IQ and emotions for the rest of your life.

In 2002, researchers using data from the Dunedin study found that cannabis use in adolescence significantly increased the likelihood of schizophrenia in adulthood, especially in individuals who had used the drug by age 15.

The research confirmed the results of an earlier Swedish study showing that heavy cannabis use at age 18 increased the risk of later schizophrenia sixfold. Studies in the Netherlands and Germany had similar findings.

…quitting pot did not seem to restore intellectual functioning in those who had been chronic pot users as teenagers, the researchers found.