Researchers from Exeter University compared obesity data in the 1980s with that of more than 300 present-day children and found signs that obesity was linked to the influence of parents for young children
Meanwhile, for teenagers more general peer-group factors seemed to be key. The researchers said this suggested different approaches may be needed to tackle the problem.
Prof Terence Wilkin, of Exeter University, who led the study, said: "Childhood obesity is one of the greatest health issues of our time. If we are to develop strategies to intervene effectively, we must first understand the cause.
"This study indicates for the first time that childhood obesity has different causes, depending on the age of the child."
His team came to this conclusion after their data suggested the rise in obesity among the very young had largely been restricted to the minority with obese parents. Rates among toddlers with non-obese parents had hardly changed.
By contrast, obesity among 16-year-olds had risen across the entire age group. Since the 1980s, obesity rates have risen from 5% to 16%.
John Middleton, of the Faculty of Public Health, said the study once again showed tough action, such as a sugar duty on fizzy drinks, was needed.
"No-one chooses to be overweight and, in fact, obesity is caused in part by the environment we live in," he added.