Well, I haven't tried pot or anything else so perhaps my frame of reference is too narrow but for what I believe to know from my own experiences I would say... (Sorry for being all serious or if this seems condescending or whatever but I have an opinion and a lot of life experience with drug addicts and seen a load of stuff I hope you never will.)
Underage use up to when the brain has finished physical development (21 yrs old I think) Is a BIG no. It is supposed to increase the likelihood of mental illness significantly in developing minds, more so in teenage males. I believe this to be true (despite wondering if the MAN is just telling us that to stop it's use) because of all the prisoners we get with mental problems who all smoke pot and are in their teens. I think if you are pre-disposed to mental illness this stuff will make you far worse, thing is you will not know until your teens which is when these problems seem to manifest if they are going to. Also the heavier the use the more likelihood of going mad. Now, I'm not talking a bit paranoid or just a bit weird I'm talking accredited mental diamond club card holder mental.
Recreationally it's probably ok in moderation like anything else. Otherwise alcohol needs to be looked at too and you'll have to prize my beer glass out of my cold dead hands, so back off. The only thing is the criminals who use it to subsidise their organisations. But if it wasn't illegal then this could be resolved. but it would then gives mixed signals to underaged users.
As for anything heavier like Heroin etc. Every criminal I knew from the start of my carrier in the Police who is a drug addict is now dead. Every one of them, young men who never lived to see their 30's and I'm talking lots of them, maybe as high as 30-40 people.