Agreed. Exercise has been shown (at least in lab animals) to preserve the myelin sheath from pathologic changes which occur with time.
Like you mentioned, I would hazard a guess that this is more a function of decreased exercise and physical activity than directly from aging.
And this quote is one of the more nonsensical statements I've heard in awhile: "That may well be why, besides achy joints and arthritis, even the fittest athletes retire and all older people move slower than they did when they were younger."
Wtf?! If they're the fittest, well by definition they're still the best. And if the quote is referring simply to aerobic fitness, I imagine most highly fit athletes don't retire due to mental slowness, but for many other reasons (traveling, lots o money, etc.)