"Overall people were 4.5 times more likely to be schizophrenic at 26 if they were regular cannabis smokers at 15, compared to 1.65 times for those who did not report regular use until age 18."

This has been proven through various means. The only way that you're gonna obtain negative effects is if you abuse you it when you're young.

A study did show, HOWEVER, that 17-20 year old light and former users had a higher increase in IQ points than non-users. But, a drop in IQ with heavy users.