THC has been shown to trigger psychosis in people with a genetic predisposition to schizophrenia. This doesn't mean reefer
causes schizophrenia, just that it can
trigger the psychosis associated with it, especially in youth. Conversely, Cannabidiol (CBD), which has almost been bred out of Cannabis, has the opposite effect (Breeders take note.). There is an episode of The Nature of Things, called The Downside of High on it. There is clinical research supporting this.
On Marijuana and IQ :
Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults]
IMO, If there is a history of schizophrenia in the family, I would say the minimum age allowed ought to be 18-19, and talk to a doctor. Other than that I wouldn't tolerate it under 16, and even then, grades need to be kept. That said, I don't actually have kids.