According to German researchers, the simple act of smiling takes years off a person's age. It is likely we consider people who are happy as being attractive in other ways - including being younger than they actually are, Daily Mail reported.
Temporary wrinkles caused due to grinning also make it more difficult to judge a person's age and so may lead to those doing the guessing to err on the side of caution.
Manuel Voelkle, of the Max Planck Institute in Berlin, asked over 150 men and women of different ages to judge the ages of the faces in more than 1,000 photographs.
He concluded: "Facial expressions have a substantial impact on accuracy.
"Relative to other facial expressions, the age of neutral faces was estimated most accurately, while the ages of those displaying happy expressions was most likely under-estimated."
The study also revealed that the older faces are more difficult to judge than the younger ones.
The age of those doing the judging was also important. In general, elderly people overestimated the age of those photographed, while younger people shaved a year or two off.
Women also fared better, with older female faces estimated on average to be three years younger than their male equivalents, the journal Psychology and Aging reports.