criminalization should be reserved to actions which cause harm to another person who didn't agree to the respective interaction.
If an adult decides to take a drug - fine. If another adult sells a drug to another adult - where's the problem?
OTOH I'd like to see the concept of mitigating circumstances removed in criminal justice. If somebody takes a drug and commits a crime - full responsibility.
On the one hand there is a considerable culture around alcohol that is difficult or impossible to expunge. Banning it simply isn't practical in a democracy where alcohol is popular across all sections of society. Hard drugs like cocaine and heroine simply don't have the cultural and social context that alcohol has, there isn't a consensus that it's acceptable.
Arguing that, well alcohol is dangerous too is a bad argument. We do this bad thing so therefore we should also be allowed to do this other bad thing doesn't follow at all. Ideally we shouldn't do either bad thing. Both alcohol and hard drug use impose considerable costs on society. If society has chosen to accept those costs in one case but not the other (while fighting to limit them) well, making such decisions is what democracy is for.
Ultimately I think the libertarian argument works for decriminalising uses, but fails at decriminalising the trade. There's no inherent right to make a profit whatever the cost to others or society.
> why not just introduce age limits like alcohol
Because they don't work..
On the high side, only 30% of people who try heroin get addicted https://www.livescience.com/62701-odds-of-heroin-dependency....
Nicotine has a higher addiction rate
Why not? Do you support criminal cartels making lots of money off of them or do you think that the War on Drugs will actually get rid of that criminal element which profits from them at some point?
I don't see a problem with anyone putting anything they want into their own body. Arrest them when they commit an actual victim-ful crime like stealing but not before that.
To that end, I say sell them with regulation just like Alcohol. Keeping sales illegal means questionable sources, questionable quality and enormous profits for criminal gangs.