How old? We find with both chickens and turkeys they sometimes just stop growing somewhere between 3 and 16 weeks of age, although appearing entirely healthy otherwise, and then we just find them dead one morning. We put this down to a genetic defect (most probably heart) that the bird could sustain while it was small but with all the organs under strain due to the fast growth stage, the body can't sustain the problem any longer. If it happened to one or two other birds I'd get worried but with just one, not.