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.

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'.

A torrent of new studies during the last 10 years have explored the potential psychological and physiological consequences of this discrepancy.

One of the most intriguing strands of this research has explored the way subjective age interacts with our personality.It is now well accepted that people tend to mellow as they get older,becoming less extroverted and less open to new experiences-personality changes which are less pronounced in people who are younger at heart and accentuated in people with older subjective ages.

Interestingly,however,the people with younger subjective ages also became more conscientious and less neurotic-positive changes that come with normal ageing.So they still seem to gain the wisdom that comes with greater life experience.But it doesn't come at the cost of the energy and exuberance of youth.It's not as if having a lower subjective age leaves us frozen in a state of permanent immaturity.

Feeling younger than your years also seems to come with a lower risk of depression and greater mental wellbeing as we age.It also means better physical health,including your risk of dementia,and less of a chance that you will be hospitalized for illness.