In his second career start, Evan Gattis had three hits, following up his debut where he hit his first home run. For those who don't know his story by now, he went to a year of JUCO, then ended up in drug rehab before arriving at Texas A&M. Four years later, he found his way back to baseball at a D2 school, and he was drafted by the Braves in the 23rd round for a $1000 bonus. After raking in the minors for two years, he made the Opening Day roster at age 26. I'm not suggesting Gattis will have a tremendous career, but I am curious how many players debuted that late and had extended good careers.
There are some players that will not be included in this list, since they were delayed due to factors out of their control (race, war service, foreign leagues). The top all-time great position players have debuted earlier than the pitchers, as Wade Boggs debuting at 24 is the oldest of the sure-fire HOFers. There were a couple other greats that couldn't get into the league until age 27: Bob Johnson and Earl Averill. Averill is in the HOF, but Johnson actually compiled about 10 more rWAR throughout their careers, both encompassing the 1930's and early 1940's.
Johnson played four seasons of minor league ball until Connie Mack sold Al Simmons in '33, and Johnson was a mainstay in the A's outfield for the next 10 years. He hit .298/.395/.520 in Philadelphia, amassing about 45 rWAR. After a 3 WAR season in Washington, he had a great '44 season with the Red Sox, punishing the league's pitchers to a .324/.431/.528 line. After one last 3 WAR season at age 39, Boston released him and he never played in the majors again.
Averill had essentially the same career, except his last three years were essentially replacement level. After three years of minor league ball, Averill was the Indians' CF the next 10 years, hitting .322/.399/.542 in Cleveland. He finished his career in Detroit and Boston with the Braves in '41. After years of tiny vote totals by the BBWAA, the Veteran's Committee elected him into the HOF in 1975.
In the 40 career WAR category, there are a couple surprising names in Maury Wills (26) and Davey Lopes (27), due to their reliance on speed, one of the earliest skills to erode. Lopes was able to adapt his game more, hitting 155 HR in his career. Eddie Stanky (27) was an OBP machine, posting a .410 OBP as a second baseman in his 11 seasons. Dolph Camilli (26) was much like Johnson and Averill, but at a bit lower level and not as long. Jake Daubert (26) was a dead-ball era first baseman whose value came from hitting gaps.
Pitchers have had much windier roads to the majors, with Grover Cleveland Alexander, Randy Johnson, and Dazzy Vance debuting at 24. At 25, you have Carl Hubbell, Eddie Plank, Mariano Rivera, and Phil Niekro. At 26, there is Old Hoss Radbourn and Mordecai Brown, Joe McGinnity at 28, and Hoyt Wilhelm at 29. These are just the HOF-level pitchers, as you can include around 30 more pitchers with at least 40 career rWAR debuting at 24 or later.
As you can see, aside from Rivera, all of these names are no longer active. Rivera is the only active pitcher above 20 WAR who debuted after 25, while Bartolo Colon and Derek Lowe have each exceeded 30 WAR debuting at 24. Strangely, the active position players include a lot more 20 WAR careers among guys debuting 25 or later. The obvious comparison to Gattis is Josh Hamilton, who also debuted in a part-time role at age 26. Ben Zobrist, Kevin Youkilis, Travis Hafner, and Mark Ellis didn't debut until age 25, while Dan Uggla and Marco Scutaro debuted at age 26. Gattis may not have the preferred career trajectory of a star, but he still has plenty of time to make that happen.