NFL Career leading rushers:
name - yards
1. Emmitt Smith+ 18,355
2. Walter Payton+ 16,726
3. Barry Sanders+ 15,269
4. Curtis Martin+ 14,101
5. LaDainian Tomlinson 13,684
# of 1000 yard seasons after age 30, % of yards gained after 30
1. Emmitt Smith+ 2, 24%
2. Walter Payton+ 4, 39%
3. Barry Sanders+ 1, 10% (he retired at 30 though, only 1 season played in his 30s)
4. Curtis Martin+ 2, 26%
5. LaDainian Tomlinson 0, 14%
Then just briefly looking over some other notable names on the NFL career rushing list, Jerome Bettis, Eric Dickerson, Marshall Faulk, Edgerrin James, Marcus Allen, OJ simpson never had a single 1000 yard season after the age of 30, and all are in the top 20 in career rushing in the NFL.
So basically, unless you are one of the top handful of backs in league history, you're probably done at age 30, and DEFINITELY done by 31 or 32, virtually no one outside of Walter Payton had any overly great seasons after the age of 32.
Steven Jackson is the current active rushing leader with 11,000 yards, he has 8 1000-yard rushing seasons, but not a single one since turning 30(he's 32
now)
Lynch will be 29 when next season starts, he's got 2 good years left, tops, maybe only 1.