Complex sentence: At the age of 35 years old at the end of the 2000 season the player character automatically declares his retirement and it becomes impossible to control the players during a match from that point onwards.
Split into simpler sentences: At the age of 35 years old (corresponding to the end of the 2000 season), the player character automatically declares his retirement. Controlling the players during a match from that point becomes impossible from that point onwards.