Cannabis isn't nearly as psysically harmful as tobacco smoke. Tobacco smoke is proven to cause cancer, but no one can proove the same for cannabis. The only "harmful" thing about it is the fact that it contains tar, and smoking it releases that tar... but it's not really that bad, the same thing would happen smoking ANY kind of plant. And no, cannabis does NOT cause brain damage, and does NOT kill brain cells, it works on a completely differant principal than something like alcohol which DOES kill brain cells (your brain actually has THC receptors, and the THC in marijuana binds to them; whereas alcohol directly effects brain cells by poisoning them, more or less). It may cause short-term memory loss, but that's a temporary side-effect. If eaten, cannabis is extremely safe, and there are basically no adverse effects (though I've never used that method of intake before, so I can't speak from experience).
And yes, I'd say the overwhelming majority of people here smoke weed (including me). I'm 18.

Welcome to the site