This is just a fun little thing I did when I was supposed to be working.   I was looking for the largest improvements and largest declines at each age.   To figure the gain or loss, I used the OPS in Year 2 minus the OPS in Year 1, multiplied by the Plate Appearances in whichever season had fewer Plate Appearances.  Eddie Mathews, for example, had a .767 OPS in 1952 and a 1.033 OPS in 1953, so that’s a gain of .266, or 266 points.   Mathews had 593 plate appearances in 1952 and 681 plate appearances in 1953, so we use the smaller number, 593, and multiply that by .266, and conclude that Mathews had an improvement as a hitter in 1953 of 158 bases.  That’s a very large improvement—the largest improvement for any 21-year-old in baseball history.   We’ll consider each integer to be a base, although, since we are multiplying based on OPS, that isn’t always exactly what it is, but that’s the easiest way to describe it. 

              I didn’t figure improvements or declines for 19^th century players because (a) I don’t consider 19^th century baseball to be major league baseball, and (b) with the constant rules changes in 19^th century baseball players have enormous year-to-year performance changes, so they tend to disproportionately populate the lists.   Anyway, with that intro, here’s a summary by age:

Age 19

              38 players improved, 30 declined, 57% improvement.

Age 20

              118 players improved, 66 declined, 64% improvement.   The 64% is by far the highest improvement percentage for any age.

              The largest improvement was by Al Kaline, 1955.  Kaline improved by 169 bases relative to 1954. 

Age 21

              273 players improved, 222 declined, 55% improvement.

              The largest improvements were by Eddie Mathews, 1953 (158 bases) and Cesar Cedeno, 1972 (144 bases.)   The largest decline was by Alex Rodriguez, 1997; A-Rod lost 127 bases. 

Age 22

              615 players improved, 457 declined, 57% improvement.

              Five players improved by 100 or more bases at age 22, who were:

              Two players declined by 100 or more bases at age 22:  Hal Trosky, 1935 (134 bases) and Ron Santo, 1962 (119 bases). 

Age 23

              976 players improved, 835 declined.  54% improvement.   Fifteen players improved by 100 or more bases at age 23, led by Woody English and including Hal Trosky and Ron Santo, who both recovered from their miserable aged-22 seasons.  I’ll just list the top ten, which cuts off Santo, who would be 11^th on the list with +109:

              Santo, after gaining 109 bases in 1963 as opposed to 1962, then picked up another 97 bases in 1964, almost making the "improvement list" in back to back seasons. 

              Eight players lost 100 or more bases at age 23, led by Bryce Harper (185 bases). 

Age 24

              1,481 players improved, 1,283 declined, 54% improvement.

              Twenty-two players have improved by 100 or more bases at age 24, led by Lou Gehrig in 1927: 

              Gehrig’s improvement in 1927 is the largest season-to-season improvement in major league history, if measured by this approach. 

              Eleven played lost 100 or more bases at age 24, led by George Scott in his epically awful 1968 campaign:

Age 25

              1,871 players improved, 1,662 declined, 53% improvement.

              Twenty-five players improved by 100 or more bases at age 25, led by Adrian Beltre in his 48-homer season in 2004.   More players had 100-base improvements at age 25 than at any other age. 

              Only six players lost 100 or more bases at age 25: 

Age 26

              1,983 players improved, 1995 declined, 49.8% improved.   Up to age 25, there are always more players who improve than those who decline.   At age 26 and after age 26, there are always more players who decline than those who improve.   However, more players improved their OPS at age 26 (1,983) than at any other age. 

              Twenty-four players improved by 100 or more bases at age 26, led by Darin Erstad in 2000: 

              Seventeen players lost 100 or more bases at age 26, led by Adrian Beltre, 198 bases:

Age 27

              1,914 players improved, 2,086 players declined, 48% improved.   However, more players declined at age 27 than at any other age, simply because the population was larger at that age than at any other.  Exactly 4,000 players either improved or declined at age 27—meaning that they were active and in the majors at both ages.  There are probably a handful who played at both ages but had exactly the same OPS at both ages.

              Seventeen players have improved by 100 or more bases at age 27, led by Stan Musial and Carl Yastrzemski.  The top four guys on this list all won the Most Valuable Player Award:

              Twenty-five players declined at age 27 by 100 or more bases, led by John Mayberry:

              Mayberry’s decline in 1976 is the second-worst in major league history.   I remember that season very well.   Mayberry, only 25 years old in 1975, was second in the MVP voting in 1975, which was his third 100-RBI season.   Royals fans were hoping for a dozen more.   There were all kinds of theories floating around about what was wrong with Mayberry, but we still don’t really know. 

Age 28

              1,789 players improved their OPS (over age 27), 2,049 declined.  47% improvement.

              Sixteen players improved by 100 or more bases at age 28, led by Mark McGwire in 1992:

              Twenty-eight players lost 100 or more bases at age 28, led by Brook Jacoby in 1988: 

Age 29

              1,663 players improved,  1871 declined.   47% improvement.

              Twenty-four players improved by 100 or more bases at age 29, led by Sammy Sosa in 1998:

              Only 19 players lost 100 or more bases at age 29, led by Tony Perez in 1971.   Note that both Perez and Bench had terrible seasons in 1971, and the Reds also lost Bobby Tolan for the season with an injury.   Bench lost 128 bases that year; that was on the age-23 list. 

Age 30

              1,411 players improved, 1,779 declined.   44% improved. 

              Eighteen players improved by 100 or more bases at age 30, led by Jimmie Foxx in his 175-RBI season in 1938—still a Red Sox record. 

              Twenty-nine players lost 100 or more bases at age 29, led by Rogers Hornsby in 1926.   Hornsby had hit .400 the previous two seasons and had hit 39 homers, but dropped to .317 with 11 homers in 1926—the largest season-to-season decline in major league history, if measured by this method:

Age 31

              1,216 players improved, 1,558 declined, 44% improvement.

              Twenty-one players improved by 100 or more bases at age 31, led by Albert Belle:

              Fifteen players declined by 100 or more bases at age 31, led by George Sisler in 1924.   Since Sisler missed the 1923 season with an illness, his 1924 season is being compared here to 1922, when he hit .422.   He returned from the injury in 1924. 

Age 32

              990 players improved, 1,391 declined, 42% improvement.

              Nineteen players improved by 100 or more bases at age 32, led by Ray Powell in 1921:

              Twenty-two players lost 100 or more bases at age 32, led by Dale Murphy in 1988, who lost 176 bases. 

Age 33

              857 players improved, 1,138 declined.   43% improved.

              Only five players improved by 100 or more bases at age 33, whereas 17 players declined by 100 or more:

Age 34

              667 players improved at age 34, 951 declined.   41% improved.  Six players at age 34 improved by 100 or more bases, led by Earl Averill in 1936:

              Thirteen players lost 100 or more bases at age 34, compared to age 33:

Age 35

              503 players improved, 732 declined.   41% improved.

              Only three players improved by 100 or more bases at age 35:  Victor Martinez in 2014 (121 bases), Mickey Vernon in 1953, and Goose Goslin in 1936.    Eleven players have declined by 100 or more bases, led by Don Buford in 1972:

Age 36

              327 players improved, 551 declined.   37% improvement.   The largest improvements were by Barry Bonds in 2001 (152 bases) and Tim Wallach in 1994.    Five players declined by 100 or more bases at age 36:

Age 37

              240 players improved, 372 declined, 39% improvement.  George McQuinn, 1947, is the only player to improve by 100 bases at age 37 (111).    Six players have declined by 100 or more bases at age 37:

Age 38

              149 players improved, 253 declined, 37% improvement.

              No player has improved by 100 or more bases at age 38, although Gavy Cravath in 1919 improved by 98 bases.   (Modern encyclopedias list him as Gavvy, but he signed his named Gavy.)   Four players have declined by 100 or more bases at age 38:

Age 39

              98 players improved, 157 declined, 38% improvement.   No player has improved by 100 bases, age 39 compared to age 38.   Three players have declined by 100 bases at age 38—Pete Rose in 1980, Patsy Donovan in 1904, and Ted Williams in 1958.   Williams in 1958 declined by 111 bases compared to 1957—and he STILL led the league in batting average, on base percentage and OPS, even after the 111-base decline. 

Age 40

              61 players improved, 97 declined, 39% improvement rate.   No player improved or declined by 100 bases at age 40. 

Age 41

              33 players improved, 61 declined, 35% improved.   Ted Williams improved by 101 bases in 1960 as opposed to 1959, making him by far the oldest player to improve by 100 bases season-to-season.   Rickey Henderson declined by 112 bases at age 41, 

Age 42

              25 players improved, 36 declined.   41% improved.    No player improved or declined by 100 bases at age 42.