You are as young as you look, doctors have found. Appearance is a useful guide to longevity and can be used to distinguish those who will die young from those likely to live to a great age, researchers say.
People who look young for their age enjoy a longer life than those who look older than their years, according to a study of twins.
Doctors often use perceived age as a general indication of health, but research on its validity has been sparse.
A team of researchers, led by Professor Kaare Christensen from the University of Southern Denmark, examined whether perceived age is linked with survival.
The latest study, carried out in Denmark, was based on 387 twin pairs.
All were aged 70 to 99 at the time their photographs were taken, which were then assessed by three groups of people.
The first group consisted of geriatric nurses, who were thought to be "experts" at assessing older people's age, while the second group comprised older women, who could also be classed as experts because they were assessing their peers.
The third group was made up of young male student teachers, who were expected to be the worst assessors.
All the photographs were mixed up, with each twin assessed on a different day to their sibling.
The results of the study, published online in the British Medical Journal (BMJ), showed that a person's perceived age - how old people think they look - was linked to how long they lived.
Over a seven-year follow-up, experts found that the bigger the difference in perceived age within a pair, the more likely it was that the older-looking twin died first.
The researchers also found that perceived age was linked with an important molecular biomarker of ageing called telomeres.
A telomere of shorter length is thought to signify faster ageing and has been linked to a number of diseases, such as cancer. People who looked young in the study had longer telomeres.
The researchers, led by a team from the Institute of Public Health at the University of Southern Denmark, concluded: "Perceived age, which is widely used by clinicians as a general indication of a patient's health, is a robust biomarker of ageing that predicts survival among those aged over 70, and correlates with important functional and molecular ageing phenotypes."