Baseball players begin to learn the stretch mechanics of pitching around age 13. The throwing mechanics from the wind up and the stretch are basically the same, but the pitcher gets to the landing position from a different starting point.
Pitchers must work to be quick from the starting point to the time the ball is released, without rushing, to keep the runners from advancing and allowing the catcher to throw out any potential base stealing threats . The pitcher's release time in the stretch should occur in 1.5 seconds or less from the starting point to release.
Below are some good videos that can help you learn and teach the stretch mechanics to young players.