I think that the exponential drop has happened as of recently as the more and more polluted our environment becomes, both in relation to living quarters and food we consume.
Before this boom in "technology" the decrease was probably much more subtle and even appearing linear as the pollution would not have been a major factor.
So leave the last 20 years out of it and we are pretty much at 2000 (for simplicity)
T Levels if dropped 1% each year since the age of 30 means that at age 30 your T levels would be 135% of what they were at 60.
60 year olds were rocking 600ish
30 Year olds 800ish
Using this I made this table (chopped out the middle)
POINTS TO REMEMBER:
That is AVERAGE T Levels...not the whole range assuming T levels dropped 1-3% per 100 years.
We also started under the assumption that T Levels only drop 1% per year after the age of 30 which is the very low end of that spectrum.
We started with the 600/800 levels but people had already spent a generation in the "pollution is killing our T levels" era.