The Decode study on mutation rate and paternal age found that mothers contribute 15 new mutations, regardless of age, while men contribute (25 + 2*(g -20), when g is the average paternal age [assumed to be greater or equal to 20]. Obviously, the number of mutations per generation goes up as average paternal age increases. What about the average number of mutations per year, which is a useful number for neutral theory?
If the generation length for both sexes is equal (to g), then the average number of mutations per year is (1/g)(15 + 25+2(g-20)) = 2, independent of generation length. Very convenient for neutral-theory calculations. But this cancellation is a coincidence: if number of mutations contributed by women were higher or lower, the per-year rate would change as the generation length changed. A big difference between average paternal age and average maternal age can also change the per-year mutation rate: if the average paternal age was 42 and the average maternal age 28, the per-year mutation rate would be 2.179.