Why the age you start smoking matters as much as how much you smoke

By Priyanjana Pramanik, MSc.Reviewed by Susha Cheriyedath, M.Sc.Jan 25 2026

A massive nationwide study shows that lighting up before age 20 leaves a lasting cardiovascular imprint, intensifying the damage of smoking and raising the risk of heart attack, stroke, and early death well into adulthood.

Study: Early age at smoking initiation is associated with elevated cardiovascular disease and mortality risk in a nationwide population-based cohort. Image Credit: Gudman / Shutterstock

In a recent study published in the journal Scientific Reports, researchers investigated whether the age at which people begin smoking is independently associated with future risks of myocardial infarction (MI), combined cardiovascular events, all-cause mortality, and stroke.

Individuals who started smoking at younger ages, particularly before 20 years, had substantially higher risks of death, stroke, and MI compared with later starters, even at the same level of cumulative smoking exposure. This suggests an additional risk beyond total smoking dose as measured by pack-years, while acknowledging that initiation age and lifetime exposure remain closely related behaviors.

Smoking as a Major Cardiovascular Risk Factor

Smoking is one of the leading preventable causes of death worldwide and remains a major contributor to cardiovascular disease, including MI and stroke. Extensive evidence shows that greater cumulative smoking exposure, measured in pack-years, increases cardiovascular risk.

However, this dose-response relationship is not entirely linear, as even light smoking can substantially elevate risk, suggesting that using pack-years as an indicator may not fully capture smoking-related harm.

Importance of Age at Smoking Initiation

Age at smoking initiation has emerged as a potential additional determinant of long-term health outcomes. Early initiation is associated with higher adult mortality and morbidity, possibly due to heightened biological vulnerability during development and long-lasting behavioral effects such as stronger nicotine dependence.

Despite this, prior studies on the effects of age at initiation on cardiovascular outcomes have yielded inconsistent findings, often limited by small sample sizes or inadequate adjustment for confounding factors.

Study Design and Population

Researchers investigated whether early smoking initiation independently increases mortality and cardiovascular risks and whether it modifies the effects of cumulative smoking exposure. They used a retrospective cohort study from a national health database in Korea, which covers more than 95% of the population through a mandatory health insurance system.

The study included 9,295,979 adults above the age of 20 who were part of the National Health Screening Program in 2009 and had no prior history of stroke, MI, or end-stage kidney disease.

Assessment of Smoking Exposure and Covariates

Smoking information was self-reported and included age at initiation, current or former smoking status, and cumulative exposure in pack-years, defined as the product of packs of cigarettes smoked per day and the number of years smoked. Participants were categorized based on initiation age and smoking exposure. Additional analyses used alternative thresholds and a composite measure of pack-years divided by initiation age.

Baseline demographic, clinical, laboratory, and lifestyle data, including blood pressure, lipid levels, diabetes status, alcohol intake, physical activity, and socioeconomic status, were collected and used as covariates.

Follow-Up and Statistical Analysis

Participants were followed until incident MI, stroke, death, or the end of 2018. Cox proportional hazards models were used to estimate the hazard ratio (HR) for each outcome, adjusting for major cardiovascular risk factors and testing interactions between age at initiation and cumulative smoking exposure.

Smoking Patterns in the Study Population

Among the study population, 40.1% had a history of smoking, and nearly one-quarter of smokers had started before the age of 20. Over approximately nine years of follow-up, all smoker groups showed higher probabilities of cardiovascular events and mortality compared with nonsmokers. The highest risks were observed in individuals who both started smoking early and accumulated heavy exposure of 20 or more pack-years.

Cardiovascular and Mortality Risk by Initiation Age

Compared with nonsmokers, individuals who started smoking early and accumulated heavy exposure had more than double the risk of MI (HR 2.43), a markedly increased risk of stroke (HR 1.78), combined stroke or MI (HR 2.00), and elevated all-cause mortality (HR 1.82). These risks were significantly higher than those observed in smokers with similar pack-years who started smoking later in life.

Interaction Between Early Initiation and Smoking Intensity

A clear dose-response relationship was observed. Earlier initiation was linked to progressively higher cardiovascular risk, even after adjusting for total pack-years. Significant interactions indicated that the harmful effects of greater smoking exposure were amplified when smoking began at younger ages. These patterns were consistent across subgroups defined by sex, metabolic health, smoking status, and different exposure thresholds.

Associations with all-cause mortality were directionally similar but generally weaker than those observed for cardiovascular outcomes, particularly among younger participants. The authors note that this may reflect limited follow-up time for mortality endpoints.

Interpretation, Strengths, and Limitations

This nationwide study demonstrates that starting smoking at a younger age substantially increases the risk of MI and stroke after statistical adjustment for cumulative smoking exposure, and that early initiation intensifies the cardiovascular harm associated with heavier smoking. These findings suggest heightened vulnerability from adolescence into early adulthood, while remaining consistent with an observational, non-causal interpretation.

Key strengths include the exceptionally large sample size, long follow-up, and detailed adjustment for clinical, behavioral, and socioeconomic confounders, allowing robust assessment of interactions between initiation age and smoking intensity.

However, limitations include reliance on self-reported smoking data, potential recall bias, lack of information on smoking changes over time or duration of cessation, and unmeasured confounders such as diet, family history, and genetic risk. The cohort was predominantly male and Asian, which may limit generalizability.

Public Health Implications

These findings indicate that preventing young people from starting smoking, particularly before the age of 20, could have substantial effects on reducing cardiovascular disease and premature death at the population level.