This graph is for running, but it shows exactly what WarrenG is trying to convey:

http://www.mcmillanrunning.com/images/graph-2-mcrun.gif

Notice that Oxygen Consumption peaks out and becomes flat, but this is not where MHR is reached, otherwise humans would be running their 3k races at the same heart rate as their 800m races. IME, this is not the case.

If you are a human, these curves are typical.

middle age
 

I think what was posted earlier makes perfect sense:

"In a nutshell
Lower heart rate
Lung capacity decreases
Muscle mass and strength loss
Decresed blood flow to the brain so reaction time is reduced."

I'd be more interested in hearing from some of the older riders, older meaning to me, 40 plus or so. Its obvious to me that the 45 year old rider that trains by riding 60 miles per week isn't going to outride the 30 year old rider that trains by riding the same amount of miles. I mean all the research is fine, I am more interested in hearing from other riders that are the same age as me. 45, ouch. But what does that mean in terms of cycling? From an organized ride standpoint, does that mean I should weigh my times against all the riders or just the riders that fall within my range in terms of age? Is fast different for the 45 year old rider and fast for the 35 year old different?

Whatever is said about your body and muscles, oxygen retention etc. Riding from an emotional look at things only keeps me younger. It helps my attitude and influences everything that I do. No question about it.


jman
Add these all up and you get slower as you age but with high intensity training you can slow the process up significantly