I am 45 and a lot slower than I was in my 20's.
I think about why I can't run so fast any more and I was amazed by Joan Benoit-Samuelson's recent results.
I think there are a few circumstantial things responsible for slowing down, in addition to the biological things. First, we forget how hard we trained in the past. Second, at least when I was younger, speed was relied upon more than strength or endurance. Third, we had other young folks to train with daily.
Nowadays, we can't train so crazily. We have to train before work when we are still half-asleep or at the end of the day, after a full day's work. Nowadays, we can't do 12 400's in 62. One 62 is likely to kill me, even if downhill. I may even pull a hamstring. Also, I train alone.
I think the way to compensate for circumstantial things is to run more mileage and longer intervals, although they may be slower. Someone mentioned swimming to develop the aerobic capacity to its fullest. This is similar to what I am saying. The other big, big thing is regularity. As you age, you have to train consistently much more so than when you are young. You also have to get all the little details right, like diet / nutrition and hydration.
There are several ways to run fast. There is no magic formula.