Okay, so why would the regulations on age limit( like those imposed on alcohol) suddenly make it more difficult for minors to get if the dealers and teenagers are already willing to break the the laws against its use?
Because the regulations would be age limit, and who could grow and sell it. Why would teenagers suddenly decide to obey the laws restricting their use of it based on their age, and why would dealers suddenly respect the laws restricting who can sell it and to whom?
Selling to kids would still be legal and there wouldn't be much profit margin for the dealers to risk selling it to children. Selling to kids isn't really profitable in the first place. When I was in school the dealers we went to were other kids, and they didn't profit much more than getting their pot for free. The people who sold to them (and to adults and tons of others) are the one's who really profited. Now if pot were legal, the total illicit market would be either to undercut the legal price or to sell to children. As long as the legal price is a lot cheaper there is no incentive to undercut it (this is why you don't see people with moonshining operations in their basement). And high school kids are a pretty small part of the overall market, and if you price your stuff too long they will just wait for an older brother or someone else to get it to them near the legal price. When I was also in school, we (and I mostly mean my friends since I didn't really drink at all then and like now) paid some loser $5-10 extra for booze to get it for us, which he pretty much spent on booze as well. The reason the pot was easier to get was I knew 5 or so people who dealt it, and just one guy willing to buy it from the liquor store.
In short, dealers for children would pretty much dry up since the risk for profiting $5-10 a purchase wouldn't really be worth the 3 or whatever years in prison selling it would be worth. Teenagers wouldn't respect the law, but my guess is most people would for the small lure of cash it takes, though I would also bet finding an older brother or whatever to purchase would be relatively easy as it is for booze now.