Well, if there are three equal life periods and the average age at death is about 78 that means young is up to 26, middle age from there to 52, and old age from there to 78. After that you're ancient.
Thanks for such a generous stream of answers. It is odd how we use everyday expressions like middle-aged but have such varied ideas of what we mean (and probably inconsistent too).
Mind you, things are changing, future enquiries such as "how old was she?" will probably get the response "Whatever" ;-)
I suppose middle age is 35-40. But I would say about 60. It gets higher the older I get :-). Nobody is entitled to call themselves old until They are at least 100 :-).