As we age we produce less Growth Hormone (GH). Less GH means less muscle mass. While men can hold their cardiovascular benefits until the 5th decade without much loss, fast twitch speed, starts to decline in the 3rd decade. That is if you don't intervene.
It's already been shown that with proper training you can reduce that and keep your fast twitch muscles by doing workouts designed to keep your GH levels high. Think short intense workouts that bring in lots of lactation.
An easy analysis that highlights fast twitch loss and age is to look at Masters sprinting. You can see a gradual loss of speed with age. An interesting fact is that stride frequency doesn't change, but stride length decreases and contact time increases. The athletes lose the ability to generate force. Keep your speed by keeping your strength. Stride length is a product of your rate of force development. The other factor is as you age your soft tissue hardens, you lose your elasticity. Flexibility starts to decline.
As you age, if you can keep your GH levels as high as you can by actively pursuing short intense workouts, you stand a much better chance of keeping your fast twitch muscles. Along with concentrating on soft tissue quality, you will be powerful well into your later years.