An exhaustive and extensive study of approximately 1000 individuals over 20 years recently published by an international team of researchers in New Zealand proves that prolonged use of marijuana is associated with significant cognitive declines. Additionally, adolescents are especially at risk for cognitive deficits due to their changing and growing brains. Specifically, those who started using marijuana before the age of 18 suffered a significant drop in IQ.
The bottom line is that teenagers who smoke pot run the risk of suffering significant and irreversible declines in their intelligence.