I think it is a combination of factors, but the most important one is being unable to sustain the same level of run mileage due to injuries. It is not just the aging process that magically slows you down. I have a good friend who is 53 and runs sub 16:30 5k and low 34 minute 10k. He is running 50-60 mpw, has not gained any weight, and has managed to stay healthy. It can be done, although I realize that he is a rare exception. Even at his level, he usually finished 3rd or fourth in national running races. He has yet to win a national title in mens 50-54 age group.
But what are your 16:30 friend’s all-time PRs??? He might have gone 14:00 at his peak.