Developing Old Man Game is a helpful explanation for Hank Aaron's career path in baseball.
It was embarrassing for Major League Baseball back in 2007 when Barry Bonds broke the sainted Hank Aaron's career home run record, which Hank had famously taken in 1973 from Babe Ruth.
Babe Ruth went up from 43 to 51, but there were big changes in the ball; the 1918 ball was made out of old newspapers or something to Help the War Effort; after Roy Chapman got killed by a dirty ball he didn't see in 1920, they used newer cleaner balls and banned the spitter. Then they switched to a lively ball around 1925.
Lou Gehrig went from 37 up to 38 but died before he hit 40. Gehrig had some Old Man Game -- in 1927, Gehrig was like Aaron in 1959 or Musial in 1948, ripping huge line drives for a ton of extra bases (but not quite as many homers). But in the 1930s, Gehrig learned to pull the ball right down the short Yankee Stadium right field line for cheap homers. (Bill Dickey did, too.)
Johnny Mize from 30 to 32. Billie Williams from 28 to 29. Willie Stargell 33 to 36 when moving to a more homer friendly park. Hank Greenberg from 39 to 39, but retired young. Frank Howard 32 (mostly in cavernous Dodger Stadium) to 34. Joe Adcock 27 to 31. Jim Thome 40 and 40. Harold Baines 22 to 24. Frank Thomas 36 to 38. Barry Bonds from 35 to 53. Rafael Palmeiro from 26 to 40. Mark McGwire from 41 to 64. Sammy Sosa from 41 to 46. Luis Gonzales from 17 to 28 (with a peak of 57 at age 33). Gary Sheffield 32 to 34.