Ladainlian Thomlinson and Shaun Alexander had sudden drop-offs in production. Before the knee injury, patriots fans speculated about the longevity of Welker's career because of all the hits he takes.
The reluctance of the jets to keep 30-year-old thomas jones after a very productive season is another example.
Its not just a matter of age. Why do players who get hit  a lot, but who are still not all that old, show a dramatic and sudden drop-off of production?