Why was Abhimanyu killed at the age of 16?
The Devas people has decided to help the people from demons and decided that God Chandra son Varchas has the ability to helped people on earth from Demons.
Chandra was not ready to allow Varchas to avatars on earth as he did not want to be separated from his son for a long period of time. But he did not against the Devas decision. So he agreed to be born as the son of Arjuna and but not for a long period of time. He wants to return his son in the 16th year.
The Devas agreed his condition and Varchas was incarnate as Abhimanyu, the son of Arjuna. He was killed at the age of 16 when he entered the Chakravyuha in the Kurukshetra war.