When we went through each position to list out baseball's fastest players earlier this week, there was one unavoidable conclusion. All the speedsters are just so young, and that's not a coincidence.
As you can see in that article, we listed the top three fastest players at each of nine positions. Of those 27, only two have reached 30 years of age. It's a list filled with 23-year-olds like Byron Buxton and Trea Turner or 26-year-olds like Billy Hamilton and William Myers. It's youth, nearly across the board, and we're adding more data points to what we were pretty sure we already knew: Speed peaks early.
That's neither surprising nor unexpected. As studies have been suggesting for years, younger players tend to be faster than older players, and for those readers who have advanced past a certain age, you can probably speak to it personally. Age takes a toll on the body.

But many of those past studies have tried to use proxies for speed, like stolen base success or overall production, since speed data for every player wasn't readily accessible. Now, of, course, it is, with the availability of Sprint Speed, which tracks every baserunner in terms of "feet per second, in a player's fastest one-second window." The Major League average is 27 feet per second, and the range goes from approximately 23 ft/sec (slow) to 30 ft/sec (elite), and you can check out the full leaderboards right here. Nine of our top 10 haven't yet reached 30 years old. Twelve of the bottom 15 have.
So with all this new data at our disposal, let's apply Sprint Speed to player age, and see if the relationship holds up. As you'd expect, it does very well. Young players tend to be slower than younger players, and as the population ages, it gets slower. With the average sitting at 27 ft/sec, age 28 is the last time the collective group of Major Leaguers is above average, at least among the current collection of players. From 29-32, they hover at just below average, before the clear decline begins at 33.

(What about that small uptick at the end, you might ask? That's largely due to the fact that few active players actually make it to their late thirties, so there's some selection bias and small sample effects at play here. Chase Utley, 39 in December, remains above average at 27.6 ft/sec, and Carlos Beltran, who recently turned 40, is still competitive at 26.1 ft/sec.)
Let's go a little further than just a single trendline and actually separate our players into three groups, roughly corresponding to "young," "medium," and "old," based on the age breakdown above. Then, we can see how many of the players within each group are above or below the 27 ft/sec Major League average. Spoiler alert: it's exactly as you'd expect.
Percent of qualified players above MLB average Sprint Speed (27 ft/sec), 2017
Age 27 and under:             78.5 percent
Age 28 through 32:            47.5 percent
Age 33 and older:              15.2 percent
Again, that makes so much sense. Of our "young" group, nearly 80 percent have average or above speed. Of our "old" group, only 15 percent do. And of our "medium" group, it's just about a perfect 50/50 split.
There's outliers, of course. There's always outliers. Youth doesn't make Kyle Schwarber (24 years old, 26 ft/sec) fast, and age hasn't prevented Rajai Davis (37 this fall, 29 ft/sec) from displaying continually outstanding speed. Catchers can be slow at any age, though it's worth noting the six fastest (led by the athleticJ.T. Realmuto) are all 28 years old or younger. Even at baseball's slowest position, age matters.

Let's take it even a step further. Since we're into the third season of Statcast™, we have enough data to compare players to themselves. It's one thing to see how a large population of players are performing, but if age does matter, then we should be able to see a trend in individual players aging on a year-to-year basis.
Do players who have appeared in adjacent seasons (and, of course, get older) get slower? Not always. But on the whole, yes. We looked at every time where a player had two consecutive qualified seasons since 2015, which added up to over 650 pairings, and we compared. For example, Jarrod Dyson was at 30 ft/sec in 2015 at age 31, and he dropped to 29.6 ft/sec in 2016 at age 32. That's a pairing, and so is that he went from 29.6 to 29.3 ft/sec at age 33 in 2017. He's lost nearly a half-foot per second per season as he's entered his thirties.
Of those 650+ pairings, here's what we found: 148 (23 percent) lost at least a half foot/second of speed, but only 44 (7 percent) gained at least a half foot/second of speed. Remember, by definition, every one of these pairings involves a player getting a year older. If we expand it to just "lost any amount of speed, no matter how minor," then a full two-thirds of our sample did. Only two players added a full ft/sec as they aged a year, while 23 of them lost a foot/sec. It's so much easier to get slower than faster.

Sometimes, that's because of injury. Adrian Beltre's drop of 1.5 ft/sec this year is one of the largest of anyone, but he's also dealt with calf problems. Luis Valbuena (-1.3 ft/sec) has had repeated hamstring trouble. Zack Cozart (-1.2 ft/sec from 2015-16) was dealing with the aftereffects of a serious knee injury. Yoenis Cespedes has fallen from 28.6 to 27.9 to 27 ft/sec as he's dealt with repeated leg issues -- and as he's entered his early thirties.
Injuries aren't age, but that's sort of the point. Injuries are more likely as a player ages, and an injury can quickly sap speed in a way where there's not a similar counterbalance to to enhance speed. As a group, every single age pairing -- from 26 to 27, or from 33 to 34, etc. -- was down by at least a little.
And, we should note, this expands to a team level, too. The fastest team in baseball (tied with Miami) is the Padres, who double as baseball's youngest team as they rebuild. The slowest team is the Blue Jays, who also have the game's oldest lineup.
Age matters. Youth matters. It's how a slugger like Cody Bellinger (28.7 ft/sec) can rank so highly in our speed leaderboard; he is, after all, not even 22 for another three weeks. This is the validation of something you inherently already figured, that as you age it's harder to keep up with the younger competitors around you. Speed peaks early. It's hard to get it back once it's gone.