Reliever WAR according to FanGraphs:
1. Eric Gagne (2003): 4.5
2. Francisco Rodriguez (2004): 4
3. Brad Lidge (2004): 3.8
4. J.J. Putz (2006): 3.6
5. B.J. Ryan (2004): 3.4
6. Eric Gagne (2002): 3.2
7. Mariano Rivera (2001): 3.3
8. Jonathan Papelbon (2006): 3.2
8. Octavio Dotel (2001): 3.2
8. Craig Kimbrel (2011): 3.2
8. Mariano Rivera (2005): 3.2
8. Rafael Betancourt (2007): 3.2
8. Takashi Saito (2006): 3.2
Average Age: 28
Youngest: Francisco Rodriguez (22), Craig Kimbrel (23)
Rookies: 2 (Kimbrel, Papelbon, Saito)
The list represents the top relievers in baseball from 2001 to 2011 based on FanGraphs WAR. The only three on the list just getting their careers started were Rodriguez, Kimbrel and Papelbon. (Saito was 36 but technically a rookie; Rodriguez was in his second year; Papelbon was technically a rookie but pitched 34 innings the previous year; Kimbrel was in his first year.) In fact, Kimbrel is tied for eighth-best season over the past 10 years while he was five years younger than the average age on the list.
Kimbrel's 1.52 FIP and 1.94 xFIP are both second on the list to Gagne's '03 season, which should be strapped to a rocket and sent to outer space.
Go back 20 years and Kimbrel's season would qualify as 15th-best by WAR.
Go back 30 years and Kimbrel's season falls off the leaderboard due to a change in reliever usage (relievers throwing more than 100 innings in the '70s). However, among rookies, he remained tied for second-best with Papelbon, Saito and Brian Fisher. In 1986, Mark Eichhorn threw 157 innings - all in relief - with a 1.72 ERA and 2.31 FIP for 5.3 WAR. He finished 38 of the 69 games appeared in. By comparison, Kimbrel finished 64 games out of his 79 appearances. It's a different way of earning reliever WAR altogether. So if you want to judge by the new form of bullpen usage, you could say Kimbrel's 2011 is tied with Papelbon's 2006 for the top rookie season among relievers in this generation.