Most of the runners I know that ran a lot of miles in their prime are no longer putting in high mileage. Most master runners and all of my training partners from 15 years ago are running considerabley less, and for the most part are all are running much slower, whereas my half maathon time is only 4 minutes slower.
I still believe you just stated the biggest reason people face significant “age related” slowing. For many reasons, from simply not having the motivation to continue at the same level to health factors, whether running-related or otherwise, the training drops off and race results drop off with the training. You seem to be proof of that. For someone who has been running since his pre-teen years to be within 4 minutes of his half marathon PR at age 55 seems astounding but not if one considers what you've offered about your training history here and what I've picked up of it elsewhere.