I am in no way attempting to pass this off as a truth, so if anyone knows any better then please feel free to say. I saw a documentary recently that claimed youths who started smoking weed around the "critical age" (it was 13-16) have more chance of developing mental problems if they maintain that habit from that age. I've known friends when I was younger who fit into that gap. I only knew one guy who reckoned he could he could hear voices, one guy went a bit loopy and became paranoid/violent. I first smoked it when I was 16, so we're going back 6 years now. Whilst never being a daily smoker, i'd say i've never gone more than 3 months without having any since that time. I'm happy to say i've never felt anything horrible, aside from throwing up sometimes when I had too much.
I personally believe that weed just exacerbates a mental state. If you're happy and trouble free, your mind should remain fine. People who had a pre-existing mental problem, underlying or not, risk having it exaggerated by the weed.