Like alchohol, we could make it illegal for the store to sell to anyone under the age of 21. IMHO, distribution by anyone other than an authorized dealer licensed by the Govt. should also be illegal. The financial gains for the Justice system would also be significant...no more prosecutions for possession by adults will clear up the courts and jail systems of these non-violent offenders, thus saving the states money.
That's the point -- it's not working with alcohol, why do you think it would work with marijuana?
Also, legalizing marijuana gives it legitimacy. It says that it's OK, and the kids will pick up on that (as they do with alcohol and cigarettes).
IMO, with legalization, usage among teens would increase as it did in Alaska:
"The consequences of legalization became evident when the Alaska Supreme Court ruled in 1975 that the state could not interfere with an adults possession of marijuana for personal consumption in the home. The courts ruling became a green light for marijuana use. Although the ruling was limited to persons 19 and over, teens were among those increasingly using marijuana. According to a 1988 University of Alaska study, the states 12 to 17-year-olds used marijuana at more than twice the national average for their age group. Alaskas residents voted in 1990 to recriminalize possession of marijuana, demonstrating their belief that increased use was too high a price to pay."
-- usdoj.gov