You say "harmless" but I'd rather say "non toxic".
To illustrate: cannabis will not create gaping holes in your brain like mercury would, but although I'm convinced there are valid reasons to use cannabinoids for medical purposes, I'm also convinced that recreational use (especially at a young age) has a terrible impact on brain development and personnality developpment.
My personal take is that the proportion of people that subjectively enjoy cannabis at the cost of feeling okay with very bad life decisions is high enough to warrant extreme carefulness when decriminalizing it. The typical example is the stoner apathy that turns into amotivational syndrome.
Re reading your comment I see that it was mainly to get this off my chest. Hope you don't mind.
Oh, yes, cannabis does seem to have an effect on people's judgement. Though it might be a bit hard to establish the direction of causality.
I just meant that the direct damage smoking either plant does to your brain pales in comparison to direct damage the tar causes to your lungs.
> I'm also convinced that recreational use (especially at a young age) has a terrible impact on brain development and personality development.
> [...] high enough to warrant extreme carefulness when decriminalizing it.
I suspect you can get most of the benefits of decriminalizing (like removing a funding source for organised crime) whilst avoiding most of the downside you mention, by slapping on an appropriately high tax on the stuff.
The main limit is that if your tax is too high, it encourages (too much of) a black market. But I'm fairly sure there's a tax that's high enough to keep the consumption of most youngsters and poor people low, whilst still avoiding much of a black market.
I explicitly mention youngsters and poor people, because as conceived the tax is a paternalistic instrument to protect people from themselves. Rich people don't need our protection, they can fend for themselves.
But I'm not sure there's not something better. Notably because a very high tax de facto creates a black market, but even a moderate tax is actually often high enough to create a black market for poor people, which in turn are already the one paying the highest price (health wise) of environnemental diseases (junk food, tobacco, alcohol, not exercising, and cannabis).
Just learn from the lessons drawn from alcohol and tobacco.
Compared to the cost of production, many countries have quite substantial taxes on alcohol and tobacco, but there's generally not that much of a black market for eg beer. (There might be more of a black market for distilled spirits, but those are also taxed more.)
I think in germany you can start to drink at 16 but only very light stuff like cider. What I heard is that when they're 18 and have the legal right to buy vodka they know what not to do with it. In contrast to the usa where you have to wait until being 21, so you manage to get your hands on some at 16 but then it has to be worth it so you take strong stuff and end up with higher problematic consumption rates.
I could imagine benefits of limiting strong cannabis strains until some later age. It also destroys the transgressive nature of the smoking act.
It actually starts at age 14 for some stuff. And if you are with your parents (and not in a restaurant), I think it's up on them to decide, even if you are younger than 14. At least in practice.
Where I grew up, it's a fairly common tradition to let even primary school kids have a sip of Eierlikör (German Egg Liqueur) for New Year's Eve.
Britain has some interesting rules, too. I think their legal drinking age for beer in a pub is lower, if you are also having a meal with your beer.
What kills you is the tar in your lungs (and on the way there, like your throat). And you get that from burning stuff in general.