The authors wrote: "Early age at smoking initiation is associated with increased risks of myocardial infarction (MI) [heart attack] and stroke. Around 3.7 million were smokers, with nearly a quarter having started before age 20 — and two per cent before 15. Study leader Jung Hun Koh said: "We aimed to further investigate the role of early age at smoking initiation on the risk of cardiovascular diseases. Among those who began smoking before 20 and accumulated more than 20 pack-years, the risk of heart attack or stroke was 2.43 times higher than in lighter smokers. Even former smokers who had kicked the habit years earlier still faced elevated risks if they had started young.