I'd let natural stuff be legal, but taxed, and 18 years old would apply. 21 could be the cutoff, the same as alcohol. The DEA doesn't have to exist. There's no reason for extra Constitutional exceptions for them or other petty tyrants.
DEA and even the FDA. Heck we can’t afford them anyway and it might make our medical industry more efficient and cheaper. We have to get away from the idea of the “nanny state” which tries to protect us from our stupidity by taking away our freedoms.
Seatbelt laws, helmet laws on bicycles even, and yes even drug laws, even prescription drugs. They are all roadsigns on the road to hell paved with good intentions.