I don't see how age is a factor in this comparison. A knee with no cartilege us a knee with no cartilege.
Not being a doctor, I'm not sure if that's the case at all.
There's also the very real question of pain tolerance / pain threshold. Some people have very high pain tolerances, just due to the way their body is wired. Other people have low pain tolerances. It doesn't make anybody "tougher" or "weaker" if their body reacts with a more severe degree of pain than somebody else.
In other words, let's say that, to him, Brandon Roy's knees are a 4 on a scale of 10 regarding pain. To JO, his are a 8 on a scale of 10. Should JO get criticized because he won't play at an 8, just because Roy will play at a 4? Who is to say that JO wouldn't be (and hasn't been) playing at a 4 for awhile?