At what age....

This article is from 2010, but I think it's still relevant, and also falls in line with my own personal experience.

"If you must assign a five-year peak period to all players regardless of description, the best shot would be 25 to 29."  Bill James

https://www.baseballprospectus...g-the-age-27-theory/

One of the very best illustrations with answers to the question posed is to go to ST and sit in the middle of a MILB facility and watch  A, AA and AAA games going on simultaneously. If you watch for 2-3 innings, you have very clear evidence of the difference in the speed and power of the players and the impact that has on how  the game is played from level to level, which is usually consistent with age gaps (not perfect but reliable in all the MILB ST games I watched.)

The difference in speed, strength, power and explosiveness is remarkably obvious when you see and measure the AB's and innings one level  against the other. In the ST games where our son was brought up to AAA and the MLB team, he said the hardest thing was trying to slow the game down made even tougher by the adrenaline pulse speeding everything up. Last week I heard him describe 3 AB's against Randy Johnson.  While he got better each pitch of each AB, he also realized that as Johnson saw his swings get better, the pitches (and slider) also got better!

I don't think the difference has to do with physical maturity, though. Peak physical maturity is about age 23. Skill development can continue. For comparison, the average age in AA is 24.4. Good article on it from a few weeks ago:

http://www.espn.com/mlb/story/...ecomes-albert-pujols

Excerpt:

"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 decline at 23 or 24. An average runner slows a little more than 1 inch per second every year 

*      *     *

The 23-year-old's lean body mass peaked sometime in the preceding five years. His bone-mineral density too. He's at the age when the body begins producing less testosterone and growth hormone. His body, knowing it won't need to build any more bone, will produce less energy. Male fertility peaks in the early 20s, the same time as pitch speed and exit velocity."

infielddad gave some good insights.

I have season tix to a AAA team.  I sit right behind home plate.  The physical maturity you can see with your naked eye between the 1st round, 21-year old and the 28-year old is sometimes significant.  But just as important is the mental maturity.

infielddad's Randy Johnson example is a great one.  The older players can pick apart a swing...or a pitch delivery much faster than a younger one.

justbaseball posted:

infielddad gave some good insights.

I have season tix to a AAA team.  I sit right behind home plate.  The physical maturity you can see with your naked eye between the 1st round, 21-year old and the 28-year old is sometimes significant.  But just as important is the mental maturity.

infielddad's Randy Johnson example is a great one.  The older players can pick apart a swing...or a pitch delivery much faster than a younger one.

But is that a factor in the progression through the system? Do they care about that? Wouldn't older players therefore have a distinct advantage over younger players throughout the spectrum of a career (until physical skills clearly diminish) and therefore the older players coveted by the coach (college/pro). We already see that in players who are held back a year (or longer) in HS to gain a physical advantage. Just wondering if that continues to propagate as that players continues through their career in your opinion?

At first I was going to respond - no.  But I think the real answer is yes...and no.  A true "prospect" gets pushed through the system faster.  They know he will mature and they have patience for that.  

Bryce Harper to the big leagues at age 19 (if I remember right).  I saw him in AA and he was a mess in the outfield - ran in on balls over his head, for example.

To supplement what justbb noted, which I believe is spot on, many organizations push players through the MILB levels to see where they fail.  A player having success at low A by May/June will likely be sent to high A or even AA. Success there gets them another move.

But the moves have a reason and one of those is to see where they fail, why they fail and can they adjust...or not.  This is very much different than HS or college. It is all about success, failure and adjustments to failure in many organizations.  In some ways, failure is a good thing at the professional level, but only if adjustments are made which are successful.

I read a wonderful article about the All Star justbb referenced from Mitty HS, locally.  He actually got to AA but was not playing.  He saw too many guys at AA who did not play and got released. He knew his only chance was to play so he asked to be moved back to high A.  More than a few thought he was crazy. (he readily credited his HS coach at Mitty for the mindset to take this risk.) However, he had made adjustments to his swing that he needed to prove would be successful and he was not going to get that at AA. At high A, with his new swing adjustments, he got AB's and produced and at very high level leading to his becoming an All Star in Seattle within just a couple of years.