I was 17 when I started regular smoking. Had tried it a few times when younger with friends but never took it up on a regular basis. Now I wish I never did when I was 17.
At 17 I was hanging around with a guy who smoked so I bought a pack and began smoking full time. Had to hide it from the parents for awhile but I think they could tell from the smell. They both were smokers at one time.
The why is tougher. Didn't give it much thought at the time but I suppose I was influenced by my peers and I also suppose I saw it as an adult thing to be doing - right of passge into adulthood perhaps? Of corse the following decades it was because I was an addict and kept denying all the bad stuff that was coming out about long term smoking. That bad stuff only happened to other people right? Isn't that what an addict does? Deny, deny, deny?