"I made my major league debut at age 20, making 3 starts, then spent most of my age 21 season in the majors, pitching poorly (4.78 ERA) with weak peripherals, but impressing with my raw ability. It took me several years to turn this raw ability into performance, as I struggled through age 24. But when I was 25, I figured something out, winning 22 games and finishing second in the Cy Young balloting. But the next year I hurt my arm, and I was out of the majors at 29. I made a brief comeback unexpectedly at age 35, but for the most part my story was one of unfulfilled potential. Who am I?"