Following kids over a long period of time is probably the best way to understand when and why they start using a substance, but researchers would never just give marijuana to kids and have them start using regularly, as that would be unethical. So for now, we don't know how kids change after they first start using cannabis and other substances.
The ongoing ABCD study should help answer those questions. ABCD is a research effort that will follow 10,000 kids around the country — starting when they are 9 or 10 years old— for 10 years. It's coordinated out of UCSD and there are 21 different sites around the country.
For this study, researchers will analyze the schools kids are enrolled in, examine at where they live, track images of their brains, see how much they exercise and sleep, and more. They'll track everything from stress to puberty hormones to substance use. They'll also see them before and after they begin experimenting with any substances, including marijuana, alcohol, nicotine, and other drugs.