A recent study in Pediatrics that tracked the course of addiction to smoking among a group of sixth-graders found that smoking only one cigarette a month was all it took to become dependent.
In the study titled "Early Course of Nicotine Dependence in Adolescent Smokers," the researchers found that one cigarette a month or one cigarette a week was enough to keep addiction satisfied, but as time goes by, the child has to smoke cigarettes more and more frequently. So people may be addicted for more than a year before they feel the need to smoke a cigarette every day.
Read a NPR story on this research here.
Apparently, this may be true not only for tweens, but even infants... Here is a YouTube video of a 2 year old baby who has become addicted and is now a chain-smoker.