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.
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.)