Coming up on 39 here.
When I was a teenager, I rode my skateboard for hours every day.
Now my knees are a bit noisy, but not painful and I have no flexibility or mobility issues.
I take glucosamine/chondroitin every day. It has quieted my knees down a lot. The crackling
noises are greatly reduced.
Thanks to a decent weight training oriented exercise program that I stick to, I've built muscle and am truly stronger now than I've ever been before in my life, except my legs, which were at their peak strength when I quit skateboarding every day. Back then I could leg press 500 pounds PER LEG. Now I max around 250 per leg. And I still skate, but not regularly.
Keep moving and keep using everything. When you stop, you lose it.
If you're not working out regularly, you're screwing yourself.
CJ