I think this is recency bias.
Remember a few years ago when all of the RBs entering the NFL were so bad that everyone became convinced RBs would no longer be drafted in the 1st round of the NFL draft, and that the key to dynasty was to ignore RB and just load up on WRs to the point where 18 of the first 21 picks in dynasty startup drafts were WRs?
Those are the RBs that are 26-28 years old right now.
I don't think it's that 26 is the age of death for a RB. I just think it's that the guys that are in that age range right now came from the worst RB generation of our lifetimes, and there were only a few good ones to begin with.
Prior to that generation of RBs there is an unending list of stud RBs that were still studs at age 26 and beyond. Peterson, Lynch, McCoy, Tomlinson, Edge, Forte, Faulk, Murray, Foster, Charles, MJD, Turner, Gore, Tiki, Taylor, and on and on it goes.