I am not a doctor, nor do I play one on TV.
But from my own personal experience and common sense (a rare thing now-a-days), when a body ages, muscles that used to stretch easily and with no effort become less pliable. The beating muscles have taken over the years become scars, and scars, as you know, are harder to stretch.
When you run, your muscles incur micro injuries; however, they heal if you give them time to rest and recover. The recovery make your body stronger. BUT if you continue to run every day, those micro injuries don’t have time to recover and heal, and over time get worse. So, instead of getting better, you get worse!
There could be other reasons why parts of your body hurt when you run, but barring any existing injuries or other problems, the main reason is age.
Again, I am no doctor. I’m speaking from experience and common sense.
Why do you think it hurts when you run? Let me and our community know.