Cannabis more damaging to under-18s, study suggests | Science | The Guardian

Cannabis more damaging to under-18s, study suggests | Science | The Guardian

From the Abstract   to "Persistent cannabis users show neuropsychological decline from childhood to midlife"
Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education. Informants also reported noticing more cognitive problems for persistent cannabis users. Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents. 

http://www.pnas.org/content/early/2012/08/22/1206820109.abstract