Two of the most successful people I know are also the hugest pot heads I've ever met, both have advanced degrees and make lots of scratch and are in demand in their fields of choice, one in a creative field, one in the tech industry. It's obvious that pot does affect the brain (duh), but it might not actually affect a person's life so negatively. If smoking weed makes an anxious person less anxious, then have at it. If smoking weed reduces pain or stress, then smoke it up. If you are super lame and get really paranoid and then pass out every time you smoke pot (who me?) then don't smoke it.
If the point of being a teenager isn't to do stupid shit, then I'm not even sure the earth goes around the sun!