I agree that like the use of alcohol and tobacco, were Marijuana to be legalized an age restriction would be the best prevention of abnormal development. Be that as it may, the human body continues to develop well past adulthood and into old age. The negative effects of such substances don't just disappear when someone reaches a certain age, and if they're negative why bother in the first place?
We can also confirm that Marijuana, like tobacco and alcohol, may not have been proven to cause detrimental health effects in the long-term, but it has been shown to increase the likelihood of such affects to those whom are genetically predisposed to them. In essence they're playing Russian Roulette. An additional bullet is added for the use of street drugs due to the fact that their purity isn't regulated by an institution that is inspected by a third party to confirm it's quality. Add another bullet when you take into consideration the lifestyle of drug dealers and the destructive individuals that surround them, and another bullet for every other drug in a persons system at the time along with possible drug interactions, since any of their individual effects are now putting even more strain on a person's biological system. The odds weren't great to begin with and now they're worse; again, why bother in the first place?
Legalization removes most of those "bullets". That and everything about the cannabis plant is better than tobacco. Smoking it is less harmful and relaxes the lung capillaries rather than constricting them like smoking tobacco does. Hemp has tons of industrial uses. Cannabinoids, constituents of marijuana smoke, have been recognized to have potential antitumor properties.
And let us not forget the primary reason for the debate: that the use of a controlled or uncontrolled substance as a means to avoid situations within an individual's life (outside of what has been prescribed by a physician) is considered drug abuse. Abuse of any kind causes pain and mistrust to oneself and those around them because, essentially, they'd rather avoid the problems in their life than deal with them constructively like a functional member of society. By not dealing with their problems they become dysfunctional, and once disfunction starts its momentum is hard to stop. This is why so many people who lack the education or followthrough to solve problems turn to drugs in the first place.
I've heard this argument before. It too is a flawed argument. "...that the use of a controlled or uncontrolled substance as a means to avoid situations within an individual's life (outside of what has been prescribed by a physician) is considered drug abuse." that right there is why it is a flawed argument. Essentially this is saying that any form of entertainment is a form of abuse. You immerse yourself in another world or setting to escape. It's the same concept.
"...they'd rather avoid the problems in their life than deal with them constructively like a functional member of society." Everyone has a means of "escape" from life. Just because you are using a controversial method does not mean you are avoiding your problems rather than dealing with them constructively. It does not mean you are not a functional member of society.
"By not dealing with their problems they become dysfunctional..." Not all irresponsible people use drugs and not all drug users are irresponsible. I'm using the term irresponsible in this context to refer to someone that doesn't deal with their problems.
Drug usage becomes a problem when you become dependent upon it, be it a physical or mental addiction. Marijuana has been proven to have little physically addictive affects and is in fact less addictive than coffee. There is a possibility for mental addiction, but that has nothing to do with marijuana itself but rather the individual using it. Such individuals that get mentally addicted were indeed looking for an escape from reality and they became dependent on that escape. The marijuana is not at fault in this situation as such an individual could have found his escape in something legal. The people that find their escape in food and get mentally addicted is why obesity is such a problem, but that does not justify making food that is bad for you illegal.
It is completely possible to use marijuana in a responsible manner in the same way you would use any other escape from reality. You watch a movie or play a video game for fun and to escape reality, but you can be mentally addicted to these activities as well.
Just because something can be used irresponsibly does not justify banning it's usage altogether. Let us not allow the actions of irresponsible individuals to strip away the freedoms of everyone else.