Steve Reeves's diet was about 20% protein. I think Mike Mentzer had a similar % as well, although Mentzer definitely took steroids.
I just don't think this T levels declining with age is calibrated correctly. It's all based on wide samples of men who may have shit lifestyles and might even not work out at all. I'd buy it tapering off in a man's 50s and 60s, but I think when the numbers show a dip with men in their younger decades it's just lifestyle choices. If you're a man in your late 30s you can look around at other men of similar age and start to clearly see how bad lifestyle choices have added up. These T level averages don't take that into account.
I really do think if a study was done with men were continuously on the straight and narrow of no-alcohol, no shit diet, always working out, you would not really see any sort of average T level decline until old age.