Absent extended longitudinal studies, it's all going to be pretty anecdotal, and probably there are genetic differences, and some biomechanics that are inclined to suffer wear and tear more than others.
My experience, having competed from 13 up until 59 (60 in a couple of weeks), is that in relative terms I've actually improved with age.
I was never stellar - 1:57 800m; 4;20 mile; 8:40 3000m - but I competed in England in track league competition to my late 30s, then came to the U.S., where I turned to road racing, with the occasional trip to the U.S. Masters Championships, where I've been fortunate enough to place a few times.
I was never a big mileage trainer, but have always trained hard - probably over-trained as far as intensity is concerned - and kept doing speedwork.
Through my 50s, I've run age-graded bests from 1500m to 10000m, and coming in on 60, I'm still doing my longer runs at not too much over 7 min per mile.
I think it comes down to a number of variables. First would be a blessed shuffle of genetics, just have the good fortunate to have stayed healthy, probably reasonable T levels, and so on, not putting on weight (I'm lighter than when I ran my fastest 800m). Still doing some physical work - I'm a desk jockey, but have horses, which can involve stacking 100+ 55lb bales of hay.
Training factors - maintaining intensity, including some full out sprints; work on core, hips and glutes - and stride coach - to try and avoid dropping to the dreaded "shuffle."
Motivation - which I think is why some of the better runners have slowed down. When you have successfully competed at a high-level, I'm sure it's hard to be motivated to be first 55 year old in your local 5k. Personally, I've always loved competition; I love pushing myself, and am more inclined to run too hard in training if I'm not careful; and being a "never was" I'm more motivated by being nearer the top of the old guy division, looking at rankings and age group bests, and so.
Overall though, I'm far from convinced that training hard through life means you can't be a competitive masters runner in your 50s and beyond