Lifestyle and environmental problems for men
While Spermatogenesis starts in adolescence, the building blocks for healthy sperm production start almost immediately after a baby boy is born.
Studies point to a crucial "window" of testicular development that begins in the growing foetus and continues for the first six months of life.
There have been such major environmental and lifestyle changes over the past 50 years that have left their mark. However, it's difficult to prove that their has been a definite decline in semen quality as many years ago, largely because the technology to assess sperm to the level it is tested today wasn't previously available. There were also no records of sperm counts in the past, so it is hard to say definitively that there has been a decline. It may be that different factors come together to have a combined effect.
Concern about the adverse effects of environmental chemicals on spermatogenesis in adult men are, in general, not supported by the available data. Where adverse effects of environmental chemicals have been shown, they are usually in an occupational setting rather than applying to the general population.