13) Joe Kennedy’s 2004 season (Age 25 season)
For a Rockies team that tried to turn to veterans via free agency in 2004, including some Rockies of the 1990s (like Vinny Castilla and Steve Reed) but their best player outside of Todd Helton was left-handed starting pitcher Joe Kennedy, a young pitcher the Rockies acquired in the 2003-04 offseason from the Tampa Bay Devil Rays in a three-team deal.
Kennedy went 9-7 with a 3.66 ERA in 27 starts (162 1/3 innings). He had an ERA+ of 135, a FIP of 4.35 with a career-high in strikeouts per nine innings at that point (6.5 K/9). He had a 5.6 WAR as a pitcher (-0.2 as a hitter so that is factored in his total WAR)) but that pitcher WAR was 6th in the NL despite missing a month due to injury.
It earned him the Opening Day start for the Rockies in 2005 but he was traded to Oakland for Eric Byrnes and Omar Quintanilla after having a complete 180 in 2005 (7.01 ERA in 16 starts before being traded).
He passed away at the age of 28 in 2007 due to hypertensive heart disease.
12) Antonio Senzatela’s 2020 season (Age 25 season)
Antonio Senzatela was the Rockies best player on their entire team in 2020 and he will look to continue that 2020 success for the Rockies in 2021.
In 2020, he went 5-3 with a 3.44 ERA in 12 starts. He had an ERA+ of 153 with a 4.57 FIP, which suggests he had some good luck. The strikeout rate dropped from 5.5 to 5.0 strikeouts per nine innings but his walk rate dropped nearly in half as it went from 4.1 to 2.2 per nine innings.
Just looking at total WAR (2.8), that nearly would have gotten on our list as it but considering that was just in 60 games, Senzatela’s 2020 season was definitely deserving to be a bit higher on the list. If you were to extrapolate his 2020 WAR for a full season, Senzatela would have been second in WAR for any Rockies player aged 25 or under but since it was not a full 162, he finds himself at 11 on our list.