Life expectancy in the Roman Empire was 35 years of age. According to an author of the time, a person at age 10, expected to live another 35 years.*
Mentally, we tended to think the disciples were roughly the same age as Jesus. 30 years +/- 5 years. (Most Sunday School pictures depict them in that age range.) According to this Chart on Roman Life Expectancy, they would most likely be dead by 53 CE. [If they were younger, say 25 years old, their life expectancy was even less—to 51 CE!]
An alternative Chart indicates if they were 25, their life expectancy would be 57 CE, or at 30, would be 59 CE.
Simply put, by the time Paul started writing his letters, we would expect many of them to be dead. By the time of the Jewish wars, we would expect all of them to be dead.
We don’t often think about life expectancy—we have a terrible tendency to “project” our own life expectancy of 70 years on people of the time.
Yet simply put, even if the Gospels were written in 65 CE—most of the eyewitnesses would be dead by then. A fact of living in that time.
*Edited, thanks to Vinny.