Barry's 2000 yarder came when he was 29, and he chalked up another 1500 yards in the following season before his early retirement at 30. Emmitt managed 900 yards in his final season with the Cardinals at the age of 35. Gore was still averaging 3.5 per carry at 37 years old and Peterson put up a 4.0 per carry season at 35 years old.
Henry is 28 so he's probably got another couple years of peak performance as long as he can stay healthy. Pretty sure all of the all time greats at the position never had more than one serious injury and some never had one at all.