Could also just be an increase in symptoms the person already had but in minor form. We tend to think of people as being either schizophrenic or... "not," but it seems far more likely that it exists, like everything in humans, on a spectrum.
People develop schizophrenia in their early twenties. Once it develops, you have it for life. The evidence is that cannabis use at an early age makes people who are susceptible to schizophrenia unable to control it and it fucks up the rest of their life. Some people smoke a ton of pot and never develop schizophrenia. Some people who are susceptible to schizophrenia and wait until their late 20's to smoke pot or use it minimally before then are able to avoid schizophrenia.
This is a perfectly valid reason to avoid heavy cannabis use before the age of 25.
it's pretty hard to establish causality for something like this, but you have:
1) very solid evidence of correlation at a population level
2) a lot of clinical experience from doctors that patients with psychotic disorder diagnoses who use cannabis tend to do worse. and, similar stories from family members of people with psychotic symptoms who aren't in treatment.
so unfortunately I think there is something real here, it's not just people having transient bad reactions to weed and going to the ER and getting a diagnosis.
