Youth of today: MLB stars younger than ever

Allow us to present a comparison of baseball in the year 2000 to baseball in '17, and it might help shed a little light on what baseball might look like in '18.
In 2000, players aged 30 and older took 86,019 plate appearances, and they threw 17,373 1/3 innings. They contributed 43.4 percent of all Wins Above Replacement (FanGraphs version).
In 2017, players aged 30 and older took 69,110 plate appearances, and they threw 15,241 innings. They contributed 32 percent of all Wins Above Replacement.
If that looks like a massive change, it should. While we're showing just two seasons there for the moment, the production center of baseball has been skewing younger for years, and by at least one measure, 2017 had the greatest number of above-average hitting seasons from players under 30 in the history of the game. It's changed roster composition. It's changed player evaluation. It is, in part, helping to create the ice-cold Hot Stove.
That last part, at least, isn't complicated. Free agents tend not to be young, by definition. Other than Eric Hosmer and Tyler Chatwood (both 28) and Addison Reed and Mike Moustakas (both 29), all of the top players this offseason are 30 or older. Young players cost much less, and they have recently produced much more. It hasn't led to a particularly exciting free-agent market, and it's far from the only reason this offseason is stuck in neutral, yet it matters. There are compelling reasons to add an older player like Lorenzo Cain, but for the most part, teams want to get younger.
It's not all that hard to see what's happened, and why it's happened, with two easy graphs. First, let's split all players into two groups, one "29 and under," one "30 and older." Let's go back to the dawn of integrated baseball in 1947 and continue on to today. We can see that while the split was near 50-50 at first (and even that was an artificial condition created by World War II), and then was relatively steady for several decades from the 1960s to the '90s.

As you can see, in the 1990s, things started to change. Older players claimed an increasingly larger share of the value. By '98, the two sides were nearly equal to 50-50. Yet every year since '98, the trend has been steadily clear, with younger players pushing the balance higher and higher, now near what it was in the '60s. Let's just look at that part of the graph, starting with the post-strike year of '95.

The 62.7 percent of Wins Above Replacement that younger players claimed in 2017 wasn't a record, but it was higher than it has been in all but three seasons dating back to 1994. And as we noted above, it did set a record for the most great young hitters.
The way we'll explain that is to look at Wins Above Replacement, in this case Baseball-Reference's implementation of it. A 2-WAR season is considered to be "league average," but we don't want merely league average. We want above average, so we'll look at every season dating back to 1901 to find the year with the most young (defined here as "29 or under") players who had seasons of 3 WAR or more. There's a tie at the top, both coming in the past three years.
Seasons with most batters worth at least 3 WAR, age-29 or younger, 1901-2017
2017 -- 59 players
2015 -- 59 players
2003 -- 56 players
2004 -- 55 players
2013 -- 54 players
2009 -- 54 players
1969 -- 54 players
You can can earnestly say that right now, we're seeing more above-average young hitters than ever. Plenty of those 59 hitters are legitimate superstars; Mike Trout, Bryce Harper, Francisco Lindor and Mookie Betts reside here. So do Nolan Arenado, Cody Bellinger, Corey Seager and Giancarlo Stanton. So do names you wouldn't immediately think of, like Matt Chapman, Ender Inciarte and Jonathan Schoop. This doesn't even include Ozzie Albies and Rafael Devers, who had borderline-historic partial age-20 seasons. There's so much talent here.
You'll notice something else, too: Eleven of the top 12 seasons here have come in the 21st century. Part of it is expected, since the sport has expanded from 16 teams in 1960 to the current 30, so any counting stat like this is skewed towards the present day. But part of it is simply that so much strong talent has entered the game over the past two decades, and they're forcing their way into lineups. 
The numbers are clear. But why has the sport changed?
In part, it's because of what we've learned about aging. A 2015 Boston Globe study, for example, clearly showed that over a span of the 30 previous years, ages 25-29 were most likely to show a player's peak production. Other studies have suggested that the peak is even earlier. When we introduced Sprint Speed, our Statcast™ foot speed metric, we separately wrote about how the new data showed clearly that speed peaked early.
Let's borrow an image and some numbers from that piece to show what we mean:

Percent of qualified players above MLB average Sprint Speed (27 feet per second) in 2017
Age 27 and under: 78.5 percent
Age 28 through 32: 47.5 percent
Age 33 and older: 15.2 percent
It's probably also because of how we value defense, which -- whether through Defensive Runs Saved, Ultimate Zone Rating or Outs Above Average -- has taken enormous steps forward since the now-prehistoric days of the 1990s. Defense doesn't trend upwards as players age, which is why former left-side infielders like Hanley Ramirez, Matt Carpenter and Miguel Cabrera are first basemen now. We're better able to measure fielding these days, teams value it more and it tends to cost less on the market.
It's probably also because of the changes to how the game is policed. In 2006, for example, older players took 46 percent of all plate appearances -- the highest non-war season ever. That number has dropped, with some ups and downs, since, down to last year's 37.3 percent. It so happens that '06 was the year Major League Baseball instituted stringent testing for performance enhancers, and playing time and production from the older crew has continued to decline since.
You can see pretty easily that the past two World Series champions have heavily relied on young talent; just think about the lineups of the 2016 Cubs (Kris Bryant and Willson Contreras, 24; Anthony Rizzo, 26) and the '17 Astros (Carlos Correa , 22; Alex Bregman, 23; Jose Altuve and George Springer, 27). Think about the "Baby Bombers" that the Yankees have ready to go into '18 (Gary Sánchez and Greg Bird, 25; Aaron Judge, 26; plus starter Luis Severino, 24). Young, talented rosters are what every team aspires to have.
Plus, of course, youth doesn't require the large free-agent outlays and associated risk that comes with older players. (Importantly, there's a lot of desirable remaining talent on the market who could help push opportunistic contenders into the playoffs, obviously.) It's the trend we're seeing, though. Youth has taken over.