here is what I came up with....I am a good person to at least get a reasonable answer to the question if you could maintain your weight as you age how much could you slow the decline indicated my age group world records?....why? because since I hit my athletic peak around 32-38 years old, my training has been largely the same....my hours and intensity have been held pretty constant which controls a variable (training schedule variability) and keeps it constant. And I have records of my race times, work outs and weight over 30 years....
if memory serves (I could look this up specifically with training logs) my PR for the 5k was 21:04. when I was 33 or so and weighed about 165 or so...(I can confirm all this later by perusing my old training logs). In 2013 at 55 I ran 24:19 at 175 lbs....
here what is interesting: by the chart in this article based on changes in vo2 max due to weight loss you can go 1:02 faster over 5K for 10 lbs of lost weight.
http://www.runnersworld.com/weight-loss/whats-your...
So theoretically I could have gone 23:17 for 5K at 55.
which means this....
a 21:04 at 32 gets an age graded score of 61.72% or 20:54
a 24:19 at 55 gets an age graded score of 63.19% or 20:24
but if I could go 23:17 for 5k at 55 due to losing 10 lbs and weighing 165 that would have scored a age graded 19:33 and an age graded score of 66.0%
so this means that my natural slowing from age 32 to 55 would have been 2:13 slower due to other aging effects besides weight gain, only 10.5% slower in 23 years, which is significantly less than the slowing of current age group world records. This means that if aging world class athletes could keep from gaining weight and body fat as they age they could cut their slowing to about .5%/year as opposed to the current 1%/year slowing.....
I know the above is just using 1 person...me. to estimate this but it would be interesting for someone to use good science and answer this question.
Regardless, weight gain is the scourge of aging athlete and its worth the effort to try and control it or lose it. (as long as you are over 18.5 BMI weight loss will help your race times)