When does smoking addiction kick in?

I didn't realize I was addicted until I'd been smoking for 8 years and tried to quit when I was 22 and in my last year of college. I couldn't quit, that's how I knew I was addicted. Now I'm 25 and smoke about 30 a day. Otherwise though I'm a pretty healthy guy ... and kind of nice looking too.

But anyway, at what point does someone know they're addicted? My 17 year old cousin started smoking a few months ago and smokes about half a pack a day, he's worried he might be addicted -- he doesn't actually want to quit but wants to be able to quit when he does want to.