I don't really think there is a "peak" for RBs in terms of age. I'm sure they get better at pass blocking and some guys have to learn patience, but generally speaking, RBs almost seem binary. They've got it until they don't.
Chris Johnson "peaked" at 24.
Thomas Jones at 30.
Clinton Portis at 22.
Curtis Martin at 31.
It's kind of crazy that the last noteworthy fantasy season for Portis was age 27 and for CJ2k it was age 28.
In the past 10 seasons, only Thomas Jones, McGahee, Peterson, Gore, Blount, and Ricky have rushed for over 1000 yards at age 30 or older. If we move it to 29, we only add McCoy, Forsett, Turner, LT, SJax, Benson, and Jamal Lewis.
So I wouldn't hold out a lot of hope of getting more than 3 years out of
DJ.