Players develop faster in the minor leagues thanks to a development bonus. The bonus applies to players who are in the minor leagues when a season "flips" on November 2nd to the next season.
For players in Low Minors, the bonus is determined by age:
26 or older: 0
For players in AAA, the bonus is determined by their major league usage over the course of the season.
ANY player of ANY age can receive a +1 bonus in AAA, regardless of EXP if, over the course of the season, the player had fewer than:
6 games played/ 21 IP (for pitchers)
35 plate appearances (for hitters)
Once a player turns 23, there is no longer any advantage for that player to be in Low Minors...they can earn a +1 bonus regardless of whether they are in LM or AAA.
Once a player turns 26, they no longer receive a bonus in LM, but can still receive a bonus in AAA. In other words, don't keep anyone in Low Minors who is 26 or older.