As I stated above, I just don't see regulation stopping people that don't fit into the "legal mold" that makes using marijuana legal. Just like underage drinking today, kids would still find a way to "score" if they wanted some stuff to smoke. ...
I'm not suggesting that regulation will stop it completely, just that it's more effective than prohibition is.
The thing with black markets is they exist because the profit which can be made outweighs the risks of being caught. If there's a legal market for a product then that significantly reduces the profit margin for the black marketers. So the vast majority of the illegal trade go away for a product that's regulated rather than prohibited. (An example of that is with alcohol. There are still bootleggers making moonshine even today, but it's a much smaller problem than it ever was during prohibition.)
And that relates to teens getting access to regulated products as well. In a market in which everyone involved is breaking the law, people are less likely to worry about where they draw the line. So if someone is selling weed, they might not really give much thought to selling it to a minor any more than they would mind selling it to anyone else. But take that same person and put him in a situation in which it's perfectly legal to sell it to an adult, but if he gets caught selling it to a minor he can go to jail it's going to be a much smaller percentage of people that will still make the sale.
Another thing to keep in mind is that for every 'pothead that is a waste of a human being' that you see, there are hundreds of people (probably more like thousands) that also use who are very productive members of society. People tend to automatically make the determination that the marijuana is the cause for the laziness in those 'potheads', but the causation is just as likely to be the reverse for many of those people. Some people really are just lazy and if it wasn't marijuana they were using they would still find some other excuse to be just as lazy.
The anti-drug propaganda has really done a good job in indoctrinating a lot of people into thinking that the only use for marijuana is to get high and have fun. But that really isn't the case.
I'm 50 years old and I've probably known many more people throughout my lifetime that are like your brother and a lot of people that are probably much worse. I've always thought the marijuana prohibition was much more draconian than was warranted, but at the same time for many years was actually pretty anti-drug. I just saw it as a waste of time. I guess though that as I've gotten older and things like arthritis, chronic backpain, tendonitis, and all the other standard ailments that people start developing as our bodies wear out has given me a different perspective.
Since marijuana became available for medical use in California, I've had reason to look into the facts a little more. When I was younger the information was a lot harder to find, but nowadays with the free flow of information the internet has provided it's much harder for the prohibitionists to suppress information. The result of that is I'm more pissed off now than I ever have been at the U.S. government and the other nations that followed it's lead regarding marijuana over the last century.
While the DOJ still stands by there stance that there is no medicinal use for marijuana, pretty much every medical, scientific, and academic research dept is learning just the opposite. When some study comes back with information about cancer fighting properties or other beneficial properties, instead of pushing for more research to figure out how and why, the prohibitionists actually try to bury the studies and when possible will even block further research. Does that really sound like they have peoples best interest in mind?
Anyone that really thinks marijuana is just about getting high really should go read the book I quoted earlier.
Clinical Applications for Cannabis and Cannabinoids
People should also ask themselves why is it that 4 presidents in an effort to show that prohibition is justified have commissioned reports about marijuana. In each case the committees returned reports concluding that prohibition of marijuana should be ended. In each case rather than following the recommendations in the reports they commissioned, or even recommending that further research might be necessary, each of those 4 presidents chose to ignore those reports. At one time the government actually tried to enact laws that would force those reports and any other marijuana research that did not support the prohibition to be destroyed. Is that the kind of politicians you want making decisions for you? It certainly isn't acceptable to me. In fact it seems down right un-American to me, our government is supposed to be better than that.