-the avg life span of a human was approximately 40
I do not know where you got this data, but given the fact that infant mortality was extremely high during this time, the avg life span would have been definitely skewed. If a person survived the initial first years, “statistically” this person was, could it be said, an outlier, and very easily could live beyond the “avg” life span.
Be that as it may, average life span is not the issue.
The issue is the specific life span of the authors. Gospel writers are the issue, not the letters of Paul, or even the book of Revelation, since these books do not primarily record the historical life of Jesus.
Mathew, and Luke were working “stiffs” during the ministry of Jesus, so at the time of the writing of their gospels, they were somewhere in the upper 50’s / lower 60’s
Mark was a teenager during the ministry of Jesus, so his age at the time of his writing he also was in his 50’s
John was a teenager during the ministry of Jesus, and it is commonly thought that his gospel was written approx 85 – 90 AD. Therefore, he could have been as old as 70 yrs at the time of his writing.