Early smoking initiation is associated with higher nicotine dependence, even in youth.

Smoking becomes less addictive and easier to quit as people age . People who start smoking before the age of 20 are more addicted to tobacco and find it harder to quit.

The study surveyed 1,382 smokers who had visited smoking cessation clinics in Japan through questionnaires, and scored their nicotine dependence according to the answers. 30% of the participants were women.

Among the participants, the average age at their first visit to a smoking cessation clinic was 58 years.

20 is the legal smoking age in Japan. Participants were divided into two groups -- those who started smoking before the age of 20 and those who started after the age of 20 -- and measured the amount of carbon monoxide in their breath to characterize how much they had smoked in the past 24 hours.

556 smokers who started smoking before the age of 20 were referred to as "early smokers" in the study, and the rest were referred to as "late smokers."

In the study, smoking cessation was defined as not smoking in the past 7 days and exhaled carbon monoxide levels below 7 ppm (parts per million).

According to the survey data, those who started smoking later smoked 22 cigarettes a day, and those who started smoking earlier smoked more, reaching 25 cigarettes a day. They exhaled carbon monoxide levels of 16.5ppm and 19ppm, respectively.

In addition, less than half of the early smokers successfully quit, only 46%. The proportion of smokers who started smoking later was 56% successful in quitting smoking.

Participants were further divided into four groups based on the age at which they started smoking -- 17 or younger, 18 to 19, 20 to 21, and 22 or older. Questionnaire scores showed that those who started smoking at or after age 22 were less dependent on nicotine.

Study author Dr Koji Hasekawa, Kyoto Medical Center, Kyoto National Hospital Organization, Japan, said: "Our findings suggest that early smoking initiation is associated with higher nicotine dependence , even in youth. Raising the legal age for tobacco to 22 or over reduces nicotine addiction and the number of people at risk of adverse health outcomes."