They did not read or watch the ads. What was a factor was that their older brother was smoking or, as Ms Fralick mentioned, it was because their parents, or the relatives they admired the most, or because so-and-so who was a big success on the tennis circuit or so-and-so who was a rock musician was smoking.
S'ils ont commencé à fumer, c'est parce que leur grand frère fumait ou, comme Mme Fralick l'a mentionné, parce que leurs parents ou des membres de la famille qu'ils admiraient le plus, un joueur de tennis ou un musicien rock bien connus, fumaient.