Seems like the 8th year is the cliff even for RB's that have elite aging.
Laidainian Tomlinson had 7 elite years. Lynch, AP, Charles (before torn ACL in week 6) all had good 8th years and all fell off a cliff after that. McCoy also had a great 8th year (feasting on bad opponents), was just OK last year. Steven Jackson had a good 9th year but not elite.
Everyone else I checked in the modern era fell off before that for elite RB's (unless the 3 people that read this post think I missed someone)
Only guy that had more sustained excellence than that was Gore in seasons 9 and 10, but you still wouldn't call him elite in those years.
And Bell's workload is/was significantly higher than many of these guys. Also, teams aren't as dumb as they used to be and keep giving "name" RB's 250+ carries late into their careers based on previous production like in the past.