The only way to do that is to make all drugs, including prescription drugs, legal to all ages. The "under 21" group represents a large portion of the drug market (even larger with legalization), unlike alcohol or cigarettes -- large enough to form an underground economy attractive to gangs.
Furthermore, the gangs will love the fact that the United States is now THE place to manufacture and grow drugs for export to the rest of the world without being hassled by the authorities. Hell, we'll have international gangs shooting it out in our streets for control of the huge export market.
Poppy fields, acres of marijuana, coca plantations, and meth labs will form the new landscape of America. Ah, the Libertarian wet dream.
Oh, you say that won't be allowed? That we'll enforce export restrictions (you jackbooted thug, you!)? Who? Not the DEA, they're gone. Who's going to monitor the borders of all 50 states? All the airports?
Oh, and if "Dallas Opium" is being smuggled into France, do they have the right to fly over and burn poppy fields in Texas (as we do in other countries)?
Take the money out of drugs my a$$. You haven't really thought this through, have you?
That's not at all true of cigarettes. There has been a black market for them for a long time and they're more attractive than ever with the huge tax burden on them now.