i smoked weed every day starting at mid 16 all the way up to 29. through that time, i was always working. i got a union construction job at 20 years old and im still doing that. bought a house at 18 and finished remodeling it last year- now its leased to sell.
then i just recently stopped. no real reason other then money. i still take a hit occasionaly on the weekend maybe, but never addicted. because its impossible to be addicted to it. its truly the absolute most harmless drug on the face of this planet. it is impossible to die from overdose unless youre allergic somehow.
MANY more people smoke weed then you realize. it just doesnt show because it just doesnt do that much to you. people who are losers that smoke weed are just that- losers who happen to smoke weed.
the tragedy is that prohibition causes WAY more damage then the drug itself. ive been jailed 3 times for it, costing thousands of dollars. it is so stupid i dont even want to get into it here, but spitting gum on a sidewalk is far more damaging to society then firing up a bowl.
Smoking weed in of itself does not necessarily correlate to problems in life. I've known many who enjoy it and regularly enjoyed it, and lead very successful and professional lives.
But the earlier you start, the more it can impact your development.
My "21" comment isn't entirely related to development, and I should have made that more clear: the majority of the consensus in the [little] research I've read relating to chemicals and neurological development doesn't actually specify any kind of hard age, other than making a case of legalization should, at best follow that of tobacco, which is age 18. That information was one that argued "after 18, it generally cannot hinder neurological development" or something along those lines. It could well not impact even an 15 year old, however, that is pushing it and in terms of youth development, you honestly must not risk that when offering up intoxicating substances for legal purchase. That, and it really depends on the individual and genetic/personality disposition toward recreation/intoxication; i.e., how do they approach chemicals. Some people are simply prone to addiction (toward anything, illicit or not, doesn't even have to be a chemical). In short, how someone approaches neurological stimulation, and their approach to self-administration of said stimulation.
Some people also will be slow to mature in other neurological focal points, and some will be fast - that, and it definitely helps to already have a firm decision-making and information-filtering ability before you start playing around with chemicals (that ability is centered entirely in the prefrontal cortex, btw). Thus, why 21 is a good physiological basis when it comes to law.
Mind you, none of that truly matters, because only the puritans and hardcore law-abiding zealots actually follow such guidelines. Youth will get their hands on what they want, one way or the other. Partly because of that prefrontal cortex not being fully mature, and mostly because humans simply are that way through and through - some take it to extremes, but in general, if there is no harm done to anyone, we want to get what we want. If it's accessible and affordable, ultimately we will.
And as far as the notion of: "more often than not, the weed smokers I know tend to be lowlifes" ... bah. You only know the obvious ones. More often than not, you've known many people you would otherwise not suspect of using illegal chemicals (for whatever reason), yet they've been enjoying it for a good part of the time you've known them (depending on how long/age when first met, of course).
And we also have a nasty habit of only suspecting the ones that already fit the stereotypes we know - do they appear to be a lowlife? I bet they smoke weed/do drugs/get into all sorts of trouble (and hell, for those types - they usually proclaim it in some fashion, sometimes proudly).
Do they fit the stereotype of a geek, always having carried a 4.0 GPA? Are they record-setting athletes? Are they very professional and seemingly by the book? Some of those people smoke weed or enjoy other drugs, but only those with which they partake will ever know.

Your hardcore, by the book bosses? They could be tokers.
Granted, the more they fit those other categories, the less often (on average) they often do it, but they might do it less often for a variety of reasons.
And there are at least a handful of regular tokers who are highly successful and professional, who also agree with keeping it illegal.
And at this point, many don't do it entirely because of regular drug testing and/or because it's illegal and carries too much risk for their aspirations/goals and choose to no longer do it (or for that reason, have yet to even try it, but may support it).
It's pretty much where it's at today because of a vicious circle of rhetoric and bullshit.