myhandleistrain said "The National Institute on Drug Abuse has an excellent story on this. They mention that especially for "early" introduction to weed in an immature brain it changes some of the brains receptors. American Addiction Centres also has some interesting reading. While I am the first to say not every pot smoker will move on to harder drugs, I did find several stats that said between 90-97% of meth and crack users first smoked marijuana. That's either an incredible coincidence or perhaps there is something to the studies in regards to gateway drugs. "
Firstly, those studies on the immature brain were considered during the legalization process, which is why there was talk of setting the legal age as high as 25. In the end, a minimum age of 18 was set. although some places have set it higher.
I also find it unconvincing that 90% of meth and crack users first smoked marijuana. It's correlation, but not causation. Most smokers I know drink a ridiculous amount of coffee, but that doesn't mean drinking coffee will make you smoke. Both are addictive, but most people are satisfied with just drinking coffee and the high they get from that (myself i:ncluded).
I've know a small handful of people in my lifetime that have gone on to use harder drugs, and they almost invariably had mental health issues going on that they were trying to escape.