Age & Weight Grading of run times

I found this site whilst ambling my way around the back waters of the internet - the premise is that your race times are factored back to the time ran by a 25 year old weighing either 143lbs (just over 10 st) for men and 110lbs for women (less than 8 st).

http://academic.udayton.edu/paulvanderburgh/weight_age_grading_calculator.htm

It's limited to the 'classic' distances of 5k, 10k, half and full mara but is an interesting concept for those of us carrying an extra person on our shoulders rather than just extra years!