This is a lazy post. You can easily find papers on this in google scholar.
See e.g. doi: 10.1210/jc.2006-1375
"A Population-Level Decline in Serum Testosterone Levels in American Men"
There has also been a large decline in sperm counts. The declines seem to have started about mid C20.
At one point, sceptics were quite vocal in their view this was not real but they have gone quiet of late e.g. Professor David Handelsman.
It is interesting that because "normal" levels are based on population samples, the normal levels have been reduced in some places as a result. Levels considered normal now would have been considered seriously low not too long ago.
There is little or no interest in finding out why this is happening. Theories abound.
Estrogen mimics in foods (soy. industrial milk, grains and grain and soy based oils), pesticides, water supplies (excreted estrogen from women taking birth control). My own suspicion is that it is due to a combination of factors
Note in this space be aware that many of the studies have been industry funded and seemingly rigged to produce a desired outcome that product X "has no [statistically] significant effect." Which result is not surprising given that the study was seemingly made so small and of such a short duration that only a huge effect would give the magic p<0.05.