The study found that children born in the 1990s were up to three times more likely to be obese than the generations before in the 1940s to 1980s.
Published in the medical journal, PLOS Medicine, the research showed that, on average, post-War male babies did not become overweight until they reached 40.
In the 1950s and 1960s, men got fatter even younger, at 33 and 30 respectively.
A similar trend was true for women too. Babies born in the 1970s were on average becoming overweight at 41, compared to 48 for those born in 1946 and 44 for those born in 1958.
However, obesity started to become more common in childhood by the 1980s and children born since then were up to three times more likely than previous generations to be overweight or obese by the age of 10.
Around a fifth of boys and a quarter of girls after 1990 are obese by the time they are 10 years old. This compared to only 7% of boys and 11% of girls born in either the 1940s or 1970s.
The University College London researchers sourced data from more than 56,000 people born in Britain between 1946 and 2001.
Professor Rebecca Hardy, one of the researchers at University College London, said: “Our work shows this coincided with the onset of an obesogenic environment in the 1980s, where there was easy access to high-calorie food.
"The onset of obesity has been getting earlier and earlier and this does have implications if people are not losing that weight.
"Being overweight over a lifetime means you accumulate risks for things like heart disease and diabetes."
Eustace De Sousa, national lead for children, young people and families, at Public Health England, said: "Evidence shows that children of obese parents are much more likely to have weight problems, which is a major concern when almost two-thirds of adults are overweight or obese.
"Almost one in 10 children in Reception are obese - but what's even more shocking is that by the time they leave primary school, this doubles to nearly one in five."