I just thought of this this morning.

Dont MLB players generally peak at an age between 26 and 30? Maybe when the Astros bring up a player late at 26 years old they do so to maintain control through his peak years. They get the highest production at minimum salary and dont risk losing him until after he has reached his peak. Im not saying that this is the MAIN reason, but that it may be one consideration out of many.

A player drafted from college is about 22. Add 3 years of minor leagues and you get 25. This control issue may be an unspoken benefit of drafting players out of college (in addition to the benefit of being better able to gauge a player's potential).