Now that life expectancy in the developed world is well over 70, fear of imminent death has receded and been replaced by a dread of growing old. While our definition of old is dependent on the age we have reached - in our 20s we fear turning 30, in our 50s we fear turning 60 and becoming an old-age pensioner - we all share a horror of becoming one of those forgetful, incapacitated, ludicrous figures our parents, aunts and uncles may have become.