Question: at what age arjuna died?
Answer: Then it was the Udyoga parva and finally Abhimanyu came to field of Mahabharata war, when he was 16. Now, add that 16 to Arjuna's age, when Abhimanyu died. It comes to the same – 55 (39+16), which was the age of Arjuna, during Mahabharata war.