The younger you start smoking, the more likely you are to smoke daily as an adult, even into your 40s, and the harder it will be to quit, warn researchers.

The study, published in the journal American Heart Association, focused on smoking at an early age, using information obtained directly from children and adolescents in the 1970s to 1980s and re-contacting many of them as recently as 2018.

"Based on our data coupled with a variety of other evidence, we found childhood smoking leads to adult smoking," said study lead author David Jacobs from the University of Minnesota in the US.

For the findings, the research team analysed smoking information on more than 6,600 people (57 per cent female) between the ages of 6-19 and during their 20s and 40s, from Finland, Australia and the US.

Participants were followed from childhood into middle age as part of the International Childhood Cardiovascular Cohort Consortium.

The study analysis found that adolescents who smoked the most and children who started smoking at younger ages were more likely to be daily smokers in their 20s and were less likely to quit smoking by their 40s.

Even children who only tried smoking at a very minimal level - a few cigarettes - were more likely to end up as a daily adult smoker.

The percentage of participants who smoked daily during their 20s was eight per cent for those who first tried smoking at age 18-19; 33 per cent for those who first tried smoking at age 15-17; 48 per cent for those who first tried smoking at age 13-14; and 50 per cent for those who first tried smoking during ages 6-12.

Only 2.6 per cent of participants who took up smoking for the first time after their 20s smoked in their 40s, the study said.

Although the current study was conducted in three developed nations, the researchers believe that the results likely apply more broadly.

"Even in low income and developing countries, the societal reinforcement of smoking, the basic addictive qualities of nicotine, and the maturation of children and children's judgment through adolescence are universal," said Jacobs.

"Cigarette smoking is an avoidable health risk, and its seeds are in childhood. Cigarette smoking, even experimentally, among children of any age should be strongly discouraged," Jacobs concluded.

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