Just for curiosity, I looked up some the Nats to see how much minor league ball they had played. Everyone except, for Yost, played 2, 3, or 4 years in the minors, and Yost started playing for the Nats -- at 17 -- during WW2. The he went to war, and MLB took a few years to get reorganized. Remember the story everyone knows: Killer was signed to a bonus at 17, so he had to spend years 18 and 19 on the big league team. Then he went to the minors: 1956, '57, and made AAA in 1958. That season, a coach from the White Sox -- Nats and Chisox shared a team -- suggested that Killebrew get closer to the plate and cover everything. Harmon came back to the Senators in 1959 and hit 42 home runs.
Of the late 1990s Yankees, the best teams I have ever cheered for, I gave you the ages of the core players when they made it. Knobloch was 22. Brosius did not become a regular until he was 28. Chad Curtis came up at 23 as a regular. The DH (1998, just to pick a year) was Strawberry, who came up at 21. Tim Raines, the other DH, became a regular at 21. The rest were fill-ins, like Ricky Ledee and Shane Spencer, who never really became starters.
By the numbers, it seems typical for good players to come up and stick sometime between 21 and 24 years old.
