These is some evidence that it may harm the brains of a small minority who are susceptible for unknown reasons (e.g. damage to the GABA system). The answer will only come with more research. Perhaps there will be a genetic test to identify the vulnerable, or minimum CBD/THC ratio regulations (CBD is neuroprotective). Obviously it is reasonably safe for the majority when used responsibly. We just have to be scientifically informed about what "responsible" use is for all genotypes and ages. It's not for everyone.