I have noticed that many of those "experts" don't seem to be getting very good results themselves. Weak, slow, overweight, and just visibly unhealthy. I know that doesn't constitute real scientific evidence of anything but it makes me question whether they really know what they're talking about.

As a practical matter most of us would probably do better by ignoring the science and instead mimicking the practices of successful masters athletes. While there is a certain amount of survivorship bias, people who are still achieving sub-elite results into their 60's must be doing something right.