Believe me, everyone who wants to do drugs is already doing it. It's incredibly easy to get. Most high schoolers can get illicit (especially marijuana and ecstacy) drugs easier than alcohol.
Also, the economic costs of drugs themselves are but a fraction of the cost of the drug war. That's right, you could pay for all these druggies social costs and save money from what we are currently spending.
The younger set would grow up with less stigma attached to their use.
This is the most liberal logic I've seen. "It's for the children." First this won't stand up because it will still be illegal for those under 21. In fact, I'd recommend 10 years in prison or more with no chance of parole for anyone caught selling to someone 16-18, at least 25 years for someone under 16, and at least 2 years in prison for those selling it to 18-20 year olds.
-Furthermore, keeping them illegal creates drug gangs which in turn creates violence. Also drug dealers are encouraged to sell harder drugs to their MJ customers to increases their profits. White market forces don't work that way, as it is easier to track to whom what is being sold and less incentive to sell it to a minor.
That said, legalizing marijuana would drop the drug war costs in half. I'd be willing to drop the anti-drug war fight if it meant would could legalize MJ. Marijuana is less addictive, less harmful and less of a high than alcohol, and MJ is impossible to OD on. There is no reason it should be illegal and alcohol legal.