Researchers in New Zealand have demonstrated that those who begin using marijuana before their 18th birthday risk lower IQs that those who wait until after their 18th birthday. 
New Zealand ranks as the fifth highest nation in marijuana consumption in the world, behind the United States, whose population has the third highest consumption of marijuana, according to the United Nations.
This long-term study gave an IQ test to 1,000 13-year-olds from the same town and then this same IQ test
was given 25 years later when they were 38 years old. Out of the 1,000 participants, researchers identified 13 regular pot users and noted an average 8% drop in their IQ by their 38th birthday.
Furthermore, this drop in intelligence occurs whether or not they stopped using in later years, which seems to demonstrate that the brain damage is permanent. The researchers also noted that anyone at any age who uses marijuana on a regular basis is at risk of memory loss and impaired cognitive development.
 Meier, M.H.; Caspi, A.; Ambler, A.; Harrington, H.; Houts, R.; Keefe, R.S.E.; McDonald, K.; Ward, A.; Poulton, R.; and Moffitt, T. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Sciences 109(40):E2657–E2664, 2012. Full Text
 Moffitt, T.E.; Meier, M.H.; Caspi, A.; and Poulton, R. Reply to Rogeberg and Daly: No evidence that socioeconomic status or personality differences confound the association between cannabis use and IQ decline. Proceeding of the National Academy of Sciences 110(11):E980-E982, 2013. Abstract