At the end of every season the player data is updated, ages are incremented and contracts are decremented. Players' skills may change in this update, old players (over 31) can only weaken while young players (under 22) are more likely to weaken than improve. If young players have played 20 or more matches their skills have more chance of strengthening.
Old players or players who are not good enough may retire at the end of a season. Your players will only retire if their contracts have expired.