Adolescents who use a lot of marijuana may end up with cognitive problems - guys in a drug treatment center who had smoked a lot of marijuana (like 6 joints/day) starting about age 13,had "abnormalities in areas of the brain that interconnect brain regions involved in memory, attention, decision-making, language and executive functioning skills."
You might ask whether it's a chicken/egg problem, but researchers also found that small amounts of marijuana damaged the brains of young mice.
The researchers said "The back of the brain develops first, and the frontal parts of the brain develop during adolescence. We found that the frontal cortex is much more affected by the drugs during adolescence. This is the area of the brain controls executive functions such as planning and impulse control. It is also the area most affected in schizophrenia."
Marijuana didn't damage the cognitive functions in adult mice.
All the more reason why it should be legal for adults only.