Unless there was a rapid die off, most of the apostles would have been alive 15 years later.
So? Of the 12 "eyewitnesses" only two (Matthew and John) wrote anything. And their accounts are like night and day, and written some 50-60 years after the fact. Petrine Epistles are of much later date and are pseudoepigraphical works. We don't know what they taught, what they said, or did. We have no clue. We ware "told" some 50 years later by Luke who heard it from others in the book of Acts.
And, we don't have the original manuscripts, so we don't know for sure what was in them. In other words, God only knows what transpired in those 15 years and what was taught, but it sure wasn't the New Testament because there was no Pauline Epistles to "explain" the theology or the Gospels!