[ethat001]

To be a running back you need three things, in this order:
1) An offensive line.
2) Talent.
3) A passing game.

As your talent goes away (ie. after 29-30 years old), you need a good offensive line (and passing game) to help keep punching out yards. If you look back at those older RB's you listed above - most of them had fantastic offensive lines and decent passing offenses. We don't have either. (I'm still rooting for him though..)