Currently we strongly think that cannabis use can cause psychotic illness in some people, and that some of those would not have gone on to develop psychotic illness if they had not used cannabis.
But it's really complex.
People with schizophrenia smoke. They start smoking early, they smoke more frequently, and when they smoke they inhale deeper. EG, this, but there are lots of similar: https://www.ncbi.nlm.nih.gov/m/pubmed/25991762/
It's hard to unpick the causal directions. But the precautionary principle is sound: there's not many benefits to children of smoking cannabis, so they should probably avoid it.
But it's really complex.
People with schizophrenia smoke. They start smoking early, they smoke more frequently, and when they smoke they inhale deeper. EG, this, but there are lots of similar: https://www.ncbi.nlm.nih.gov/m/pubmed/25991762/
It's hard to unpick the causal directions. But the precautionary principle is sound: there's not many benefits to children of smoking cannabis, so they should probably avoid it.