Cannabis use is associated with an earlier age of psychosis onset, according to an article in Schizophrenia Bulletin, and daily use of high-potency cannabis is associated with an average of 6 years earlier onset than that of non-users.
Forti, M., Sallis, H., Allegri, F., Trotta, A.; Daily Use, Especially of High-Potency Cannabis, Drives the Earlier Onset of Psychosis in Cannabis Users. Schizophrenia Bulletin. Online December 17, 2013. doi: 10.1093/schbul/sbt181