3. Sandy Koufax
Sandy Koufax's first six seasons in the majors were the very definition of mediocrity. From 1955-60, he never had a winning season, with an ERA closer to 4.00 than 3.00. His command of his pitches could be charitably described as shaky, with more than five walks per nine innings.
But his hometown Brooklyn Dodgers stuck with him, because he had this blazing fastball, that when harnessed, would be pretty unhittable. So when they moved to L.A. in 1958, they brought him along, using him mostly as a fourth starter or long reliever, in the hopes that he would find a cure for this wildness.
Koufax would reward their faith in spades, beginning in 1961, when he led the National League in strikeouts while finally cementing his spot in the rotation. But it was in 1963, after having turned 27, that he became one of the all-time greats. In a four-year span, he would lead the NL in ERA every year and strikeouts in three of the four, win at least 19 games each season, claim three Cy Youngs and an MVP, lead the Dodgers to three World Series, and pitch four no-hitters.
And just like that, he retired after the '66 season, at the age of 30, because of excruciating pain from arthritis. A late bloomer, to be sure, who also walked away at the zenith of his career.