A pitcher's development is not just a function of age. It is also a function of experience. What you will find very few pitchers who appreciably improved the level of performance after they have pitched 1,000 big league innings. It is so rare as to be anamolous. The problem is that Ben like you probably looked at age as the developmental factor without looking at his big league experience. By the time a pitcher gets to 1,100 big league innings, he is what he is, and Porcello is a back of the rotation pitcher.
With regard to the examples you listed, by the time Glavine had thrown 1,000 innings he had led the league in wins twice and had already won a Cy Young Award
By the time Kevin Brown pitched 800 innings, he had a 20 win season and an All Star selection.
Randy Johnson? The only question about Johnson was whether he could consistently throw strikes. And even with his developmental issues due to being 6'10". By the time he had as many innings as Porcello, he was a huge star and had 2 All Star game selections.
Drabek had most of his best years before he threw 1,000 innings.
And Pedro, by the time he hit Porcello's current experience level, he had 2 Cy Young's (should have been 3) and 4 All Star game selections. He was already a megastar.