Rodgers was more like 27 when he really broke through into the 2:09-2:10 range. Really hard to compare a marathoner who took two years off, smoked a pack a day, and led the life of a hippie consciencious objecter to a professional runner.
A couple things about Salazars group. He's big on gadgets and biomechanics and things like that. All that stuff about Ritz and his form that came out a couple years back. AlSal leaves no stone unturned, so maybe they tweaked Mo's runnig form and training and taught him to be a fast closer.
Alternatively maybe these athletes are on supplements of some type. But so far advanced as not to be testible, let alone on the radar as something banned. In theory there could have been blood doping going on in the 1950s or 60s. It's not that hard to take out a pint of blood. Store it for a few months and re-inject it. But nobody (in the public) was even thinking about it until the 1970s.
Finally. I pr'd at 8K at 32, running more than a minute faster on less miles and easier training than I did at 20-22. And even faster than at 25, when I pr'd for 10K and marathon. It does happen.
Then again I won't be surprised if something does come out in the open some day. Saddened and disgusted but not surprised.