Former Yomiuri Giants star Warren Cromartie and I had an argument the other day about when ballplayers reach their prime, and since I tend to be a know-it-all, I looked to see what the evidence suggests. After looking to find out who actually has been the best player in Japan since 2020 and when Japanese players reach their peak, it turns out we were both kind of wrong.
Our discussion began with his evaluation of the relative strengths of two players who appear headed for a switch from Japan’s majors to MLB, the Giants’ Kazuma Okamoto and the Yakult Swallows’ Munetaka Murakami.
Former Giants manager Tatsunori Hara put Cromartie in uniform a few years ago so he could help Okamoto find his feet, and boy did the youngster respond.
Okamoto will be 29 on June 30, has played seven full seasons. He is on track to be an international free agent after the 2026 season, and Cromartie said he has reason to believe the Giants will post him this autumn.
Cromartie asserted that Okamoto is a far better player and that while Murakami was better in his 2022 triple crown season as a 22-year, old, Okamoto was the best player in Japan over the past five years and would get a bigger MLB contract. This last part may well turn out to be true, but given that he’s a right-handed bat, I wouldn’t want to bet the house on it.
I argued that while MLB scouts like both players because of their power swings, Murakami has a higher established ceiling and has a far better chance of improving from where he is now because of his age than Okamoto.
While there is no guarantee that Murakami will ever find his way again, he has proven able to compete at a level few in Japan have ever matched, and time is on his side. I argued that given his age it would be highly unlikely for Okamoto to improve much upon what we’re seeing now.
Cromartie disagreed, asserting that Japanese players don’t reach their prime until after they are 30, to which I responded that very few players are better after 30 than before. I respect Cromartie’s experience and his up-close view of players who were extremely good after 30, but he’s talking about trees and I’m talking about forests.
So that led to two questions:
- Who has been Japan’s best player over the past five seasons?
- When do Japanese players reach their prime?
Japan’s best player since 2020
If Cro had argued that Okamoto was better than Murakami in three of the last five seasons, he would have been correct. Okamoto was a hair better in 2020, and a win or two better in both 2023 and 2024, as measured by Bill James’ win shares. But the sums of their estimated contributions on offense and defense, have not been close.
Since 2020, Okamoto has not only not been the best, but Murakami by virtue of his huge 2021 and 2022 seasons, has been by far the best. It’s easy to see how far Murakami has fallen, but those two seasons, but Murakami’s 2024 season was much closer to Okamoto’s best in 2024, about one win, than Okamoto’s 2024 season was to Murakami’s second best.
| Year | Player | Age | Win Shares |
|---|---|---|---|
| 2020 | Okamoto | 24 | 21.7 |
| 2020 | Murakami | 20 | 20.0 |
| 2021 | Okamoto | 25 | 20.7 |
| 2021 | Murakami | 21 | 32.9 |
| 2022 | Okamoto | 26 | 20.6 |
| 2022 | Murakami | 22 | 48.2 |
| 2023 | Okamoto | 27 | 25.9 |
| 2023 | Murakami | 23 | 20.8 |
| 2024 | Okamoto | 28 | 26.3 |
| 2024 | Murakami | 24 | 23.9 |
Here is a table with the rankings since 2020. If we made them since 2023, Okamoto would still fall short in wins created for his team compared to Kensuke Kondo and was a close third behind Koji Chikamoto. Each win share represents one third of a team win.
I’d meant to have a database table here, but the data table plugin I use is currently boycotting even my simplest asks…
| Player | Win Shares 2020-2024 |
|---|---|
| Munetaka Murakami | 145.8 |
| Kensuke Kondo | 130.1 |
| Hideto Asamura | 122.4 |
| Koji Chikamoto | 118.3 |
| Yuki Yanagita | 117.7 |
| Kazuma Okamoto | 115.2 |
| Tomoya Mori | 102.1 |
| Yusuke Oyama | 98.0 |
| Keita Sano | 94.1 |
| Toshiro Miyazaki | 93.9 |
Prime time
One difficulty when thinking about aging curves for ballplayers is opportunity. Looking at ordinary players doesn’t tell us much about how players’ abilities improve and degrade over time. Ordinary players’ careers often fall victim to opportunity rather than ability, while outstanding players generally play as long they are healthy and productive.
I looked at Japan’s 62 most productive position players through 2023, those who created at least 200 win shares, or 63-plus wins over the course of their careers and never played in MLB. I further whittled that group down to only include only the 51 players whose careers ended after 1965.
If a player reaches his peak after the age of 30, we would expect to see one or both of two things, that he plays more games after 30 or is more productive per game. To measure this, I took the four-year average of each player’s percent of his team’s game, and his win shares per game for his seasons from age 27 to 30, and from age 31 to 34.
Surprisingly enough a surprising number of Japanese stars have been a tiny bit better after 30, more than I expected. Here’s how they broke down:
- 7 played nearly as well in somewhat reduced playing time.
- 11 played nearly as much or more but at a lower level of production.
- 15 played nearly as much or more, and were nearly as good or better after 30.
- 18 did not play nearly as much after 30 and were not nearly as productive.
A couple of notes are worth making.
- Of the 15 players who were better after 30, we had a pair of outfielders from Sendai’s Tohoku Fukushi University. Kazuhiro Wada played corporate ball before spending years as a backup catcher with the Seibu Lions and didn’t get regular playing time until he was 30. The other, Tomoaki Kanemoto, was platooned with the Hiroshima Carp until he was 28.
- Kazuyoshi Tatsunami and Tomonori Maeda were both far better before the age of 27 than after 30, so their peak years weren’t really after 30. So the real total of great players who were definitely better after 30 is something like 13 out of 49 or even 11 out of 47.
- Sadaharu Oh, Japan’s greatest player, was better after the age of 30, not a LOT better, but the best stretch of his career came from ages 31 to 34.
From this study, I would conclude that most Japanese players reach their peak before they turn 31, but that a little less than a third actually manage to get better with age up to a point.
