Who really wants a world where we would find that out?
Better to do the conservative things and avoid drugs, do something noble and contribute in this world rather than self medicating.
That would be up to states and localities ... presumably around the same age as for alcohol purchase.
and how often would drugs be used
As Ken H poiints out above, drug use is higher now than when those drugs were legal.
"do something noble and contribute in this world rather than self medicating"
"A wise man ought to realize that health is the most valuable possession and learn how to treat his illnesses by his own judgement.
Hippocrates - A regimen for Health circa 500 B.C.
If by 'noble' you mean possessing superiority of character or of ideals or morals than I believe the contributions of the drug war to the world fall far short of such description. Moreover, no greater nobility exists than self sacrifice in defense of vulnerable innocence. A war is being waged by the greatest nation on earth againt an herb gifted by God to man and beast alike at the very beginning of time. This gifted herb's protectors, possessors and propagators are the casualties and victims of an unjust and unconstitutional war waged by the federal government upon its own citizenry. A war which has its origin in corporate manipulation of markets through government power wages on today because the most powerful of lobbies standing against this herb remains in power, OIL. There is no nobility to be found in treasonous acts perpetrated by black masked and black armored DEAmen busting down doors in the middle of the night and dragging off citizens to the largest prison population in the world for possession of a gift from God in a free country. One may not choose what herb to consume yet children are sacrificed by the thousands daily in this country in the name of privacy and choice regarding one's own body. Others fight and die to spread this type of 'freedom' and we wonder why the enemy says it is all about oil.
Furthermore, if you think it is noble to sacrifice your own personal sovereignty to the oligarchy of doctors than do so. But, attempting to force any other to do so in this nation should be treason. This is one reason socialized medicine is such a dangerous prospect.
You never answered me as to if you think the Second Amendment should be interpereted literally or if that has also become outdated.