I'm only 37 and already feeling it. A few weeks ago, I spent 4 hours straight on my knees, putting together a new dresser for my soon-to-be-born son. I was constantly kneeling, twisting, shifting and turning while working on it. A day or two later, my right knee felt awful - it hurt to bend and move, depending on how I was sitting or walking.
I determined that it was probably a form of burstitis (called floor-layers/roofer's knee) where people who spend a lot of time on their knees burst the fluid sacs around the joint, causing pain. I could walk (so it wasn't some sort of ligament damage), but it was so uncomfortable. It went away after 2-3 weeks and is mostly back to normal now.
Other than the aches and pains of doing physical work, I seem to be in very good shape and health still.