These changes were not seen in the twins whose diet included meat, eggs and dairy products.
Some experts suggested that in the long term a vegan diet may lead to nutritional deficiencies and may not be suitable for all age groups, while others said the differences observed in the study could be due to weight loss.
By the end of the study, the researchers found that estimates of biological age based on levels of DNA methylation (a type of chemical modification of DNA used to estimate biological age) had declined.
Biological aging refers to the decline in function of tissues and cells in the body, as opposed to chronological age (the number of years a person has lived).
Previous studies have reported that elevated DNA methylation levels are associated with aging.
But Varun Dwaraka of epigenetic testing company True Diagnostics and Christopher Gardner of Stanford University in California said it was unclear how much of the difference between the pairs could be attributed to differences in diet.

Tom Sanders, professor emeritus of nutrition at King’s College London in the UK, who was not involved in the study, said the study did find some differences in how vegans age.
He noted that this did not take into account the fact that vitamin and mineral deficiencies often don’t show up until several years later, or that some research suggests a vegan diet may be worse for the health of older people.
Dr Duane Mellor, dietitian and spokesman for the British Dietetic Association, said that although the study compared vegan and omnivorous diets, the diets were not perfectly matched in terms of calories.
“It’s possible that the reduction in energy intake changed the way the participants’ DNA changed,” he explained.
The findings are published in the journal BMC Medicine.