It does stop kids from being openly advertised drugs and makes it difficult for kids to get drugs. That is the whole point of legislation, not to eliminate but mitigate.
I can assure you the average teenager in a western country has absolutely zero issues getting their weed. In fact, the older you get, the harder it becomes (as your social circle tends to shrink once you have a job).
In theory yes, in practice no. It's smarter to legalise it, it cuts off income source for criminals, creates taxable income that can be actually used to fund help centers etc.
That's very easy - go to area of train station, or some nearby park, and look for immigrant-looking people just standing around, trying to keep eye contact with passers or saying quietly "bonjour".
95% of the time you will get exactly what you want, otherwise it would be some form of "desolee". This is obvious, everybody knows and sees it, en masse (as in you are choosing from tens of such people in given area).
The problem with this kind of thinking is that you're funelling large streams of money into pockets of criminals. You're also not solving anything, in practice people keep buying it - without actual minor controls (unlike ie. alcohol or tobacco) and with blurred perception (ie. same dude probably deals weed, cocaine, heroine, whatever). It is shifting market to black side with all negative consequences that come with it. It creates this disgusting alternative, unofficial layers in cities where law enforcement long ago gave up on enforcing this ban and ordinary people learn/know to stay the fuck out and ignore.