Taking the average age at death of Americans to be roughly 75 years, and
the average age of ferrets to be roughly 7.5 years, then the ratio of 10
human years per ferret year would be about right.  The problem is that the
initial part of the ferret ageing curve is different than that of a human,
so the first few years of a ferret's life can be quite different than
10 times human years.  For example, at the end of the first year of a
ferret's life, they are full grown, capable of reproduction, and living
independently.  This places them at about 20 years when compared to a
human.  Like in humans, during the middle years there are little changes
in the ferret's attitudes or behavior.  This starts to change at about 4
or 5 years, when ferrets begin to play less and sleep more.  Gradually,
ferrets will begin to slow down, will lose their position of dominance,
lose most of their interest in play, and sleep for long periods of time.
This relates to humans at about 60-70.  By 8 years of age, many ferrets
show obvious signs of old age, including changes in bowel habits, lack of
tolerance for food changes, and a lack of self care.  Some even seem
forgetful, especially of the toilet location.  This roughly corresponds
to about the 80s in humans.
 
While the beginning of the 10:1 scale is wrong, I do think that a five year
old ferret roughly corresponds to a 50 year old human, a six year old
ferret to a 60 year old human, etc.  In other words, after five years, I
think 10 human years are roughly about the same as a year in ferrets.  Here
is a rough scale *I* use (based upon established human ageing parameters,
compared to similar applicable signs of ageing in ferrets):
 
Ferret/Human
1/20
2/27
3/38
4/43
5/50
6/60
7/70
8/80
9/90
 
There are three important things to consider.  First, my scale is based
on published signs of ageing and NOT upon experimental data.  In other
words, it is possible=97perhaps probable=97that a controlled study may
find my scale inaccurate.  Second, all studies depend upon statistical
generalizations and do not necessarily apply to specific instances.  In
other words, remember that ferrets are just like humans in terms of ageing
in that not all members of a species age at the same rate.  Simply put,
not all 50 year old humans look or act 50; most seem about 50, some seem
older and some seem younger.  This is normal, and ferrets are no different.
Third, and most important, while the national average age of death for
male humans is about 77 years, deaths around that mean are not normally
distributed.  Rather, they are heavily skewed, with a slow, long rise,
a moderate plateau around the mean, then a rapid fall.  This type of
distribution has relatively few deaths prior to the "national average",
with a lot of deaths after the mean.  For example, in a normal
distribution, for every death at 50, you would have one at 104 (assuming
a mean of 77).  However, in an aging demographic, for every death at 50,
you will have about 27 dying at 78 to maintain the average.  In terms of
ferrets, that means relatively few die before 7.5, but at lot die after
that age, which is why ferrets older than 9 are very rare (and why humans
reaching the century mark are likewise so unusual).
 
Species have evolved to die after a few breeding seasons and when their
offspring are self-sufficient.  This does two things.  First, it maximizes
resources for the offspring, insuring their survival.  Can you imagine the
success rate of offspring attempting to enter a landscape dominated by
older adults who have learned where all the good stuff was hidden?  The
youngsters wouldn't have a chance!  Second, when natural selection weeds
out the less-fit individuals, the resulting population is well adapted to
the environment.  If the population was skewed to older individuals and the
environment changes, then the entire population could die and the species
becomes extinct.  However, if the population is dominated by younger
individuals, prior to when natural selection can weed out the less-fit,
then there will be some individuals that can adapt to environmental
changes, and pass those traits on to their offspring, thus insuring species
survival.  From an evolutionary standpoint, limiting the time an individual
can live might be really bad for the individual, but it is really great for
the species.
 
Bob C
[Posted in FML issue 3495]