x1**2 + x2**2 + ... + xN**2
   population_stdev = sqrt(---------------------------)
                                       N

   xi = age_i - average_age

##</code><code>##

                           x1**2 + x2**2 + ... + xN**2
       sample_stdev = sqrt(---------------------------)
                                     N - 1

   xi = age_i - average_age