I found this post:
Full of references, and the most important take-away is:
“the average 22 year old male today has an average testosterone level roughly equal to that of a 67 year old man in 2000”
From all the links above, I found the one by Carnivore Aurelius the easiest to read to quickly find out what you are doing wrong, and if you live in a Western country, you are probably doing a few things wrong.
The author Justin deserves a medal for digging up this information!