now, the question is that are you getting better at distance as you get older? or is it just that you are running it more often. i can see people PRing at the 800 at like 21 or 22 because if they went to college, that might be the last time they're running it, and would they be able to have run a better marathon at a younger age.
take bekele for example, its probable he'll go up to a marathon when he gets older (following his idol) and probability is that he'll get the marathon world record (dont bash, but think about it, it is very probable) would he run faster now in the marathon, or should he really wait and take out the short distances now before he has a few more thousand miles under his belt (we may never know, just a question to really think about)