Suggests because it's difficult to prove it conclusively. But let's get real, it's common sense that being a heavy cannabis user would make someone more spaced-out and perform worse on memory tasks. Excessive chronic use of any type of drug is going to have detrimental mental and physical side effects.
HOWEVER! Studies have shown marijuana can have detrimental effects on developing brains over time. I started using it after this point in my life, and I do NOT encourage those under 18 to use it regularly (probably best to start after ~20-21 years). I can attest that using it regularly before that age is more likely to lead to mental issues (apathy OR, in some cases, anxiety and depression).
Still, before coming across like an ignorant ass, you might want to consider regular users like myself that enjoy life, work hard, and do very well for themselves.
Marijuana and how it affects users and the world around them is not something simple that you can just generalize.
Perfect - thank you for that point.
A developing brain, that is where the damage is done regarding Marijuana.
What part of the brain does early use of Marijuana end up changing? The last section of the brain to mature: the prefrontal cortex. Studies have shown the average age where the prefrontal cortex reaches maturity (no longer developing) is 25 years old. Some older, some later.
Marijuana use in the 20s probably should be mostly fine - you've had a history of solid uninterrupted growth (in that part of the brain). Marijuana use in the mid-teens, however... it's a recipe for potential problems. No guarantees someone will have the "who they are" change, and there is no real set final outcome; some may see it change them negatively (if starting early, this is more often the case if anything), some may actually see beneficial growth. Maybe the experience and thoughts gave them a push in the right direction - and they started in that direction immediately.
The prefrontal cortex, responsible for many things, is the primary focal point for decision-making. Altering how this part of the brain develops can impact personality and strength of will.
There is some truth to the stereotype, but the road there is highly variable and only some find themselves traveling in that direction. Overuse, especially when young, can lead to a sort of "cementing apathy" - but there has to be something there to start with.
Their personality at the start was already looking to take a certain shape, they were probably weak-willed, not motivated strongly about anything in the future (no specific drive - though this might not be true for all "stereotypical potheads"), and most importantly, they probably have a style of addictive personality.
I identified that (addictive personality) fairly early on in my life - it helped that my genetics demonstrated a perfect example of this behavior in an uncle, so I had a "this is what taking the left fork in the road looks like" situation.
Without such "flaws" going in, there's far less likelihood that marijuana use will negatively impact who you are at the physiological level. That said, you can easily get yourself into an apathetic state without drugs if conditions were right. Life situations and various things can throw mentally competent people into a tailspin, so there are never any guarantees in life about what you will become based on what you choose to do or not do.
As a man would identifies as pro-marijuana, I fully agree that any legal marijuana should be limited to 21 and over. While I see no evidence that 18y/o is any worse for alcohol than 21y/o, there is a large enough body of evidence to enforce the notion that young adults must not be granted legal use of marijuana - for our future, all of ours.
Better education will be important though, because I thoroughly support people completely ruining their own lives due to their own choices, so long as they are fully informed. If we can get everyone in high school to fully understand marijuana's potential impact on the brain, potentially worse than most other chemicals they can get their hands on while they are still at that age. (except for the harder drugs - I'm sure heroin is worse for a young brain than marijuana

)
There's much to be had, beneficially, for marijuana's legalization. Not only, in an adult's hands, can it help keep the body from getting too strung out on stress and better able to help relax, and even naturally target inflammation.
In the body of a fully mature adult, marijuana will be a better NSAID than basically all NSAIDs, even the majority of the stronger prescription ones. It's equally as effective, and less habit forming, than prescription opioids.
It'll also help protect us from who knows what random soon-to-be-labeled carcinogenic chemicals to which we've been routinely exposed. How? Research has demonstrated that marijuana has an anti-tumor impact and can help kick the ass of the immune system to get it fighting the enemy it didn't see at first. I think this was specifically THC that had anti-tumor properties; it alone isn't all that wonderful of a chemical though. Many have complained due to only receiving THC (in the form of Marinol, a prescription dose of pure THC), and that's because all of the negative effects of THC are usually mediated by the balance of a whole host of other cannabinoids, cannabinol and cannabidiol as examples. There are many, many other chemicals in the plant, all of which are either without effect alone, or very small. But by acting in various ratios on the different types of receptors, they can add to or subtract to the THC alone is doing. Each have different affinities for the two types of receptors. How each receptor gets played, at what strength and frequency, determines the final impact. That's why different strains, and especially the different species/cultivars, have different end-state "highs" for the user: some will make you stoned with "couch-lock", with more cognitive activation (introspective, philosophical), whereas others will make you bouncing off the walls looking for entertainment.