Yes, earlier onset. Research results seem to show that marijuana (pot, mj) can start a person's schizophrenia sooner than if he did not use mj. A person, especially a teenager (maybe because his brain is not fully developed), with a family history of schizophrenia or other psychosis, should know his family history and avoid experimenting with mj or other illegal drugs that might have a similar effect on the brain.
No. However many teens who have schizophrenia are mis diagnosed early on in their illness because they attempt to self medicate.