As someone who was once a young weed smoker, I get the sense that this is more correlation than causation. I know as many failures as I do successes from this position.

Those who have been failures were going to be failures anyway irrespective of whether they smoked weed or not, as was the case for those who ended up successful.

With that said, I don't deny that weed smoking at a young age would likely affect your brain negatively. But I don't think it determines what your outcome will be.