Imagine, for a moment, that you had no birth certificate and your age was simply based on the way you feel inside. How old would you say you are?

Like your height or shoe size, the number of years that have passed since you first entered the world is an unchangeable fact. But everyday experience suggests that we often don’t experience ageing the same way, with many people feeling older or younger than they really are.

Scientists are increasingly interested in this quality. They are finding that your ‘subjective age’ may be essential for understanding the reasons that some people appear to flourish as they age – while others fade. “The extent to which older adults feel much younger than they are may determine important daily or life decisions for what they will do next,” says Brian Nosek at the University of Virginia.

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Its importance doesn’t end there. Various studies have even shown that your subjective age also can predict various important health outcomes, including your risk of death. In some very real ways, you really are ‘only as old as you feel’.

Given these enticing results, many researchers are now trying to unpick the many biological, psychological, and social factors that shape the individual experience of ageing – and how this knowledge might help us live longer, healthier lives.