In 2013, about 28 percent of all Wins Above Replacement were created by the under-25 set. That was the ninth-largest share for any season since 1976. Output from youngsters has been on the upswing since the mid-to-late 1990s, when the percentage of WAR from young players hit its nadir. That nadir happened to occur at the height of baseball’s so-called steroid era.Further research is needed on the subject, but recent work on aging curves in the post-PED (performance enhancing drug) period suggests that today’s players are better earlier in their careers, and then decline much sooner and more sharply.Given that, it’s no surprise to see the game’s fresh faces producing a larger share of wins. A few years ago, Bill James — baseball writer and pagan god of sabermetrics — was asked whether baseball was in the midst of a golden age of great young stars. James created a “young talent inventory” (pay-walled), a somewhat elaborate methodology to measure whether runs were being created or saved by players aged 25 or younger.I’m not going to delve into so intricate a technique. But I like the 25-or-under cut-off (as do other prospect-watchers), so I decided to add up the Wins Above Replacement (WAR) produced by players in that age range for every season going back to 1976, the dawn of the free-agency/arbitration era. (Before then, player movement was far more restricted than it is now.)What proportion of total WAR in all of Major League Baseball has been generated by young players?