Does running on pavement damage the legs or does it make the muscles more responsive

Stay on grass, your knees will thank you later

I now run 100% on trails. The reason is, your stride pattern is always changing, whereas with running on the roads you can do exactly the same stride pattern for miles. Running on the trails builds lower body core strength, which is good for my back. I used to run for a month and then visit the chiro to straighten things out. Now that is no longer necessary, although I still go the the chiro every month to get things stretched out. I keep him on retainer, just in case. He said all the lateral motion builds strength where I need it to protect my back. When I was younger I ran 100% on roads.

Only if it's well kept. Any potholes or unevenness and he's gonna roll his ankle sooner or later...

Soft dirt is still my favorite surface for easy runs because of that.