I am 79 now and have been running for over 4 decades. (I did not start until my late 30s. If I had started earlier my speed would have been faster for years, until about the time I actually did start,) After initial improvement for about 4 years, my pace slowed, year after year.
Loss of muscle fibers has been stated as a cause. However, I have continued to do strength training every other day, and the weights have not decreased. Runners continue to use their leg muscles.
I have read two other possibilities: (1) loss of fast twitch muscles and (2) Obfuscation of the fast twitch muscles so that they resemble slow twitch.
I don’t believe my stride is shorter. Actually, I’ve read that a stride can be too long. It’s the frequency of the strides that determine speed. Too long a stride results in fewer strides per minute. I’m fairly sure that my strides per minute have decreased. That could be from loss of elasticity. But why do you lose elasticity as you age?
Stride length (as long as it’s not overstriding) and frequency determine speed.
An easy check for overstriding is that your footplant is directly under your center of gravity.
You can lose stride length and still maintain frequency. Since your legs don’t have to go as far per stride, you can keep frequency while losing speed.