Interesting post by Nate Silver at BP Unfiltered.  He briefly looks at whether pitchers actually improve after age 21 (unlike hitters, who pretty consistently improve up to age 26-27).  The natural conclusion is that when you have young pitcher tearing up the minor leagues, you might as well get him up to the majors, since there’s no guarantee that you’re not seeing his “peak” years, anyway. 

The flip side is that Silver specifically identifies Bailey as an example of a small sub-class of pitchers who do see improvement with age (“guys with good stuff, high strikeout rates and highish walk rates”).