Some of the characters in the Mahabharata lived long lives. Even Arjuna was about to become a grandfather when the great battle was fought in Kurukshetra. Bhishma his grandfather's age. Remember that Devavrata had been a young lad when he took the terrible vow that allowed his father to marry Satyavati. Satyavati's grandsons were Dhritarashtra, Pandu, and Vidura. So Bhishma was even older than Arjuna's grandfather would have been!
Dronacharya loved his son, Aswhatthama, too much. That love drove many decisions Drona took in life. Ashwatthama, on the other hand, loved his own life too much. That much we know because Bhishma says as much on the eve of the war. "He loves his own life too much. This brahmana always wishes for a long life. [Ch 164, Udyoga Parva]", and after the war, Ashwatthama confesses, "I was scared of saving my life. I released the weapon out of fear. I was scared of Bhimasena.[Ch 15, Aishika Parva]"