They grow up so fast! Beijing Olympics star Shawn Johnson, just 16 when she captured America’s heart in the gymnastics competition, announced her retirement from the sport Sunday at the age of 20. Johnson is one of the most decorated American gymnasts in history, with three silvers and one gold medal from Beijing in 2008, along with three gold medals from world championships. Two years ago, Johnson injured her knee in a skiing accident and feared she'd never compete again. Her torn ACL and MCL turned out to be too hard to practice on. "This is a really hard moment," Johnson said. "I still have the heart, drive and desire to compete. Unfortunately my road has been cut a little short." She plans on starting college in the fall of 2013 at either Stanford or Vanderbilt.