From Trout to Pujols: Inside MLB's aging curve

23.

There are two outs, the bases are empty and the batter is 23.

He is, as he steps into the batter's box, the most exciting player in baseball. As a rookie, he has hit a ball 513 feet in batting practice, thrown a pitch 102.5 mph from the mound and reached a top sprint speed -- nearly 30 feet per second -- faster than three-quarters of his peers can touch. These are the highlight years, and it feels like just the beginning.

But an athlete's physical decline begins before most of us notice it, and even the 23-year-old body can do things today that it might not be able to do tomorrow. Fastball speed starts going down in a player's early 20s, and spin rate drops with it. Exit velocity begins to…