O.K. I'll probably touch on some things that have already been said, and may be vague on some, but since I do a kind of "stream of consciousness" thing, that's bound to happen

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Depending on where you get your statistics roughly 40% of youth (under 21) have used pot. The thing here is that pot dealers don't ask for ID. They sell to whomever has the cash.
If pot were legal, taxed and regulated, many of the dealers would just quit dealing since they do it partly to get their own supplies.
Another reason would be because the Feds and Staties would be able to go after them on tax charges. That's how they finally nailed Al Capone ya know. Tax evasion.
Now I know that's certainly not going to keep it from those kids that want it, hell I used to swipe my dad's smokes and nips off of his whiskey bottle when I was 12. The same would be sure to happen with pot.
Now, with this tax money coming in, you start a drug education program. And not that BS they used to give in the 60s and 70s either. Lay out the facts about the contents of marijuana smoke (way worse than tobacco)and other drugs, have some addicts tell about their own personal Hell, show a few more of the gory highway patrol movies to help it sink in. And make operating a vehicle under the influence (of any drug) a felony with a heavy fine involved as well as jail time. And mandatory sentencing if one causes a death or serious injury.
Let adults make their own choices, just like helmets.
It's a crime with no victim, at least it would be if the business was taken from the criminal element.
It makes absolutely no sense to put someone in prison for a graduate degree in criminality for smoking dope.
Also we could free up the courts, LEOs and prison system to deal with the violent offenders that should never take a free breath.
I'm sure those who want to privatize the penal system oppose legalizing anything that might take money out of their pockets, but that's a different subject.
And for those of you who think that only the dopers want to legalize it, I'm not.