We are living in baseball's age of youth. Baseball's two foremost stars, Mike Trout and Bryce Harper, could still be on their parents' insurance plans, and behind them are ranks of exciting, fresh faces who are both very young and very good. As Dave Cameron pointed out last week at FanGraphs, young players are producing more WAR/PA than ever before in baseball history.
Cameron was building off research done by FiveThirtyEight's Rob Arthur, in which he concluded the driving force behind this trend was simply that young players are better than ever. This is a significant change from the steroid era of the late '90s and early '00s, when performance enhancing drugs allowed players to remain healthy and productive well into their 30s and 40s, but it's also a change from the rest of baseball history.
It's possible this is all a fluke, a random clustering of talented players, and things will return to normal soon. But if we assume it isn't, and that this shift toward young players is real, you would think that teams would have adjusted and become more aggressive with their minor leaguers, knowing that there's a better chance than ever before they represent an upgrade over roster spots currently held by veterans.
I used Baseball Reference's Play Index to find every debut position player season from 1996 to 2015, even those with only one or two plate appearances. Every team that called up these players had never seen them against major league pitching. Then I calculated the percentage of plate appearances by those players that were at a certain offensive level, as measured by OPS+.
It fluctuates a lot, unsurprisingly, but while the middle buckets have remained fairly stable, the percentage of debut plate appearances below an 80 OPS+ has fallen sharply, and the rate of plate appearances above a 120 OPS+ has risen sharply. This might look like it's measuring basically the same thing as the above chart from FanGraphs, but it's not. These are players in their first season, who are coming up without any major league experience and immediately hitting 20% better than average. Here's how the overall OPS+ of players in their debut seasons has changed over the last 20 years:
There are year-to-year fluctuations, but also a clear upward trend, from 74 in 1996 to just shy of 100 in 2015. Speed and defense generally peak early in a player's career; the average debuting player in 2015 was therefore probably better than his counterparts at running the bases and manning the field, on average, and he was seemingly an equally productive player at the plate.
New major leaguers have evidently gotten better and better, and the trend has shown no sign of reversing itself in recent years. The cause of this trend is, at least in part, the surge in the abilities of young players, but that surge has combined with a lack of adjustment on the part of MLB teams. Here's the average age of a player making his debut over the last 20 years:
Players are better, younger, than in the past, but they're having to wait essentially as long as they always have in the minors before getting called up. To borrow a favorite metaphor of Baseball Prospectus Editor-in-Chief Sam Miller, this is like how never missing a flight means you're showing up to the airport too early. By waiting this long, teams are reducing the number of below-average debuts by players not yet ready for the show at the cost of leaving MLB-ready players in the minors, presumably making the parent club worse as a result.
There are a few plausible explanations, all of which are probably responsible to some extent. First, teams might just not have adjusted to the new, youthful norm yet. This is the most obvious one, as MLB teams are not particularly nimble when it comes to changing their assumptions and norms. If we see this trend change in the next few seasons – if players start getting called up at a lower level of performance, or at a younger age – it would be good evidence for this theory.
Second, this might be yet another pernicious effect of the current CBA's approach to young players. Teams have shown extreme willingness to keep major-league ready players in the minors in order to squeeze another year of labor out of them at below-market rates. If teams think young players are peaking at the same age, they might be disinclined to call them up earlier, even if they're ready for the majors, as doing so would allow that player to hit free agency before he's had his best years. That might strike you as foolish, unfair to the young player, or boring to the fan, and I would probably agree, but teams are both slow to change their behavior and relentless in their pursuits of profits. If this is a big factor behind this trend, I'd guess we'll have to endure good players wasting away in the minors for several more years.
The final possible factor, and the most legitimate in my mind, has to do with the psychology of calling up a young player. If a prospect is already good enough to be a positive factor on the major league roster but has the potential to develop into a bona fide superstar, a team might be justifiably hesitant to call him up. If this prospect hits 10 or 15 percent below average, it might be the first time in his entire life he's been anything other than great at baseball, compared to the players on the field with him. If a team thinks this player might not take that well, leaving him in the minors to develop (both as a player and as an adult) might be a wise decision. This one is extremely tough to measure, which is one reason a lot of sabermetric writers (myself included) might be inclined to dismiss it. It's easy for me to see players as random number generators, but quotes from players in this situation are a good reminder that they're real people. Teams that keep that in mind are not only more humane, but probably better off in the long-run.
Still, with the current crop of young players as good as they are, teams are probably letting valuable resources go to waste somewhere on their minor league roster. The only question remaining is how long that continues to be the case, and how quickly teams can adjust to the new normal.
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Henry Druschel is a Contributing Editor at Beyond the Box Score. You can follow him on Twitter at @henrydruschel.