There is no age at which it becomes a good idea to take any recreational substance, with the possible exception of a few psychedelics (LSD).
I didn't even need to look at your forum title

Cannabis is a psychedelic, fyi.
I don't quiet understand the rest of your post - it seems to jump back and forth from cannabis having an individualistic (YMMV) effect depending on outlook, mindset, environment, or nurture - to cannabis promoting laziness or effecting test scores regardless.
The "damage" thus inflicted varies considerably with regard to one's life situation... It is ultimately dependent on the judgment of the individual...
v.s
The main problem that arises with weed (in my personal experience, i.e. not data) is that it makes you ignore any obligation that you don't understand. If you know that you have to, say, pay the electic bill, lest the power to your house be cut off, weed usually isn't going to prevent you from doing that. If you don't understand why getting a good score on the SAT is important for your future, you're going to smoke and not study, because, like, who cares, man?
While I agree that college, a good education, and a solid drive to, you know, *do things* are important, I don't think that college is for everyone. It's really just a fact. I might be misunderstanding your point, but I don't think so. The last sentence is a little confusing, though.
A 14-year-old smoking cannabis is almost surely better off than a 32-year-old taking clonazepam...
Err. What? I'm not sure exactly how the two are even comparable, or how a grown adult taking klonopin is recreational... whereas a pre-pubescent kid smoking cannabis is most certainly recreational. In either case, the two are entirely different drugs. It's a lot like comparing LSD to china white, it dun' make no sense.
I'm willing to bet that 90% of the kids reading this thread in high school are fully capable of getting a degree in Something That Matters from A University That Doesn't Suck. Unfortunately, I'm also willing to bet that most of you won't.
I don't think the 90% statistic works well here. I'd be fairly confident in saying I know over 10% of regular CD posters who DO have a degree or are currently going for one. In either case, that doesn't have much to do with what im discussing so I digress.
The easiest way to explain this to a kid is something along the lines of "weed can make you lazy; don't smoke when you need to do things.
Here we agree, well on the basis anyway.
Not knocking your post, I just don't follow the logic completely. Drugs interact with everyone differently. For instance, I know several people who smoke to control psychosis, schizophrenia, and bipolar type 1 and 2. On the other hand, I know people who cannot smoke because it exaggerates their symptoms. Same token - I study and write more clearly while high - not the case for my housemate, who can smoke a bowl and then get sucked into playing world of warcraft for 5 hours without studying for a final. Flip side, I can't think of any other drug, besides d/l-amps, that help my concentration in the same way that cannabis can.
That is not to say I smoke before studying/writing religiously, because I personally like to limit my consumption. If I can, I prefer to smoke when my responsibilities are done, but as a neuroscience/philosophy dual-major with two federal work studies, sometimes the responsibilities do not end. Not everyone is alike.
Regardless, smoking at 14 is too young. I started smoking when I was 13 I believe, and it truly did not impact me until I started paying bills. A horrendously large amount of factors influence how an individual will behave with most drugs. I agree with your non-cannabis related thoughts though - that teenagers (hell, even a lot of adults) are less likely to know what is really important and what isn't. The world isn't so black and white, though.