In order to show the effects of cannabis clearly in a research study, it is of course best to have a prospective, randomized, controlled experiment, conducted over a long period of time.
This would not be ethical in humans. In fact, I don't see that it was particularly ethical in monkeys either. But Verrico, Gu, et al. did such a study, published in the April 2014 edition of The American Journal of Psychiatry, giving adolescent rhesus monkeys daily IV doses of THC 5 days per week for 6 months. A control group, matched for baseline cognitive performance, received IV infusions with no THC.
They found significant impairments in spatial working memory in the THC group.
This is strong evidence that marijuana has negative effects on cognition in adolescents. It did not prove that there are lasting cognitive deficits after the THC has been metabolized out of the body.
We can conclude from this study that daily heavy THC use in otherwise healthy adolescents is likely to interfere with optimal cognitive performance, which could impair schoolwork and possibly contribute to cumulative risk of various other developmental deficits.
The study does not address risk to cognitive function in adults. And it does not address the possibility that THC may be useful for managing other symptoms for some individuals, despite the side-effect of spatial memory impairment.