There are several people I know that have been affected by weed negatively, a couple more than the other.
Friend A started smoking weed maybe around 13-14 years old and began smoking it all day, everyday when he was 15 till about 16. I considered him as a very intelligent person, he still is, but he's definitely not like he used to be.
Friend B started smoking weed around 16 years old, he still does, and has actually grown a physical dependence on it.
Friend C started last year, he's 19, after him and his girlfriend of 2 years split up. He was extremely social, charming, and all the good stuff when it came to social interactions. He entered depression and now smokes weed everyday and cannot seem to enjoy life unless he's high. His intelligence definitely took a hit, possibly because he's high all the time so I have no way to assess his sober cognitive functioning, and he's definitely not as social anymore. I have difficulties interacting with him since he's always high. He claims that he's not addicted, or that he's dependent on it, or that he doesn't use it to 'escape', but he clearly does it for all those reasons. He's still smoking as of now, and I'm not sure when he'll stop.
Now for myself I've only smoked weed twice and it induces severe anxiety. On one of those occasions I hyperventilated and passed out -- I felt out of it for about a week. I cannot smoke weed.