Future Tien was killed by Future Android 18 in the alternate timeline depicted in Dragon Ball Z.
The prologue to the TV special reveals Future Tien's demise. He is shown being swiftly defeated by a punch to the stomach delivered by Future Android 18. This event, tragically, ended Tien's life in that timeline. The Androids were rampaging throughout the world, causing widespread destruction and death, and Tien was one of their victims. At the time of his death, Future Tien was 34 years old. The Androids 17 & 18 were much stronger than any Z-Fighters. Future Tien was unable to win the battle against the Androids.
