Which would imply a "middle age" of around 18 years - which was nothing near what I said, so what are you disagreeing with?
Perhaps my chosen wording was poorly defined - "middle age" can be a subjective social perception, and that would be based on several factors, not all relevant to this discussion. If a person was not crippled or die from disease or virus, or as a result of violence or accident, or from famine related causes, or from some work in an unknown-danger profession (lead, mercury, etc.) and had good health and luck in general (no cancer/etc), they might well live to be in their 70's or older - but most simply were not able tot dodge all those dangers. So, what is "middle age" when those conditions for a "natural death" are simply too hard for most to meet?
Estimates for medieval life expectancy seem to vary (see "statistics"
), but (generously?) assuming that the average life expectancy after 21* is about +25-35 years** or so, then that means an average of not much over 50 - and so, yes, really, a "middle age" of 25-30.
[i](* i.e. once a person reaches 21 years, how much more they'll live, on average)
(** This seems supported by some admittedly shallow web research: hyw.com/books/history/Fertilit.htm
sarahwoodbury.com/?p=453
wiki.answers.com/Q/What_was_life ... eval_times)[/i]
(If you include infant/child mortality, "middle age" drops at least 10 years, depending - but most sources seem to agree that the non-aristocrat who lives to be 60+ was the exception, not the rule.)
And that's "life expectancy", not "productive life expectancy". So, assuming a 13th century person works at their job for maybe 30 years might be more reasonable.
(Of course, if you are of that ilk of folk who, today, believe that in the western world "middle age" starts around 45-50 regardless of the fact that we have very few nona- and centegenarians around, then we simply have different working definitions of that term.) 