Think that might have something to do with the constant rise in people eating junk food? Theres more and more fast food places opening up all the time and people are drinking things like energy drinks and fizzy cola beverages more than ever. None of these things help with T-levels. How does a doctor test for it? I assume its a blood sample?
It has more to do with our general lifestyle now.
Where i'm from in the 50's you probably worked down the mines or built ships, proper manly jobs requiring intense amounts of physical labour every single day from 18 til you literally couldn't do it anymore.
If you think about all the things that boost testosterone; physical exercise, sex, losing your temper, eating red meat etc they're all masculine activities. Logically no fighter should have "low" testosterone and i doubt they are actually "low" by normal standards unless they have abused steroids and as a result their body can't produce a normal amount anymore.