I started at 16, which seems to be the age where most people begin around here, I remember when I was 16-17 and was attending our version of college... good times...
There is a small risk of developing an addiction, however an addiction won't mess your life up as much as with other drugs like Cocaine or Heroin. I was one of the few that got addicted for a long time though, I've spent 5 years of my life smoking weed every day and didn't care about school anymore etc. I also spent a lot of money on weed in those five years, around €15.000.

Some people have an addictive personality I guess (like myself). However due to the fact that weed is legal to buy here I never came into contact with 'harder' drugs, so the stepping stone theory is pure BS IMHO.
THC binds at several points in your brain and mimics a neurotransmitter called Anandamide, it also indirectly increases the release of Dopamine by interfering with the release of GABA neurotransmitters which normally inhibit an excessive release of Dopamine. This allows more Dopamine to be released.
Smoking weed can adversely effect your short-term memory and your coordination, however these effects are not permanent and the destruction of brain cells has not been proven in the case of Cannabis.
If you want a (very) detailed explanation about how Cannabis works or want some basic information about neurochemistry I will post it. If its good, it could be stickied perhaps?

LOL haha... nah seriously I have some very detailed information about Cannabis which I would like to share with everyone who smokes, I could create a FAQ even if there is interest.
Peace o/