In what might be called the mainstream American culture, which includes exposures to synthetic chemicals, childhood vaccines, standard American diet
First generation: (born around 1960 plus or minus a few years) Parents were not exposed to synthetic chemicals until they were adults. Anecdotally there seem to be a few more obese kids, geeky kids, gay kids, kids with allergies, kids needing orthodontics, etc., but nothing too alarming and kids seem to grow out of some of these problems. The birth rate for this generation falls a bit when adulthood is reached but nothing too dramatic. There may be a few years decrease in life expectancy becoming evident.
Second generation: (born around 1985 plus or minus a few years) First transgenerational effects may be seen, in other words, parents who were exposed to these things during their own fetal and childhood development start to have kids. In addition to what plagues the first generation born around 1960, hyperactivity, attention deficit disorder, autism, more severe allergies, diabetes, and other disorders become more common and less curable. Some are now common enough to have names and to be screened for. This generation sees a dramatic fall in their birthrate upon reaching adulthood.
Third generation: (born around 2015 plus or minus a few more years) Now going out on a limb. Transgenerational effects are now quite noticeable and influence the culture and economy to a great extent. There are lots more lesbian, gay, bisexual, and transgender adults compared to generations born before World War II, so many in fact that an acronym to describe this phenomenon is thought to be needed, LGBT. It is rare for a child not to have some kind of disorder. Disorders become even more severe; for example whereas many children born in 1960 may have been a bit geeky, now many are severely autistic. The birth rate for this generation takes a dramatic fall to less than half of the birthrate of the generation born before World War II.
Fourth generation: (born around 2045 plus or minus a few more years) Now really going out on a limb. Transgenerational effects in the mainstream American culture are now so severe that fertility and lifespan are affected to the point that it is evident mainstream culture is no longer viable. In most cases, disorders are so severe as to be irreversible and in many cases untreatable.
While the periphery breaks down rather slowly at first, the capital cities of the hegemon should collapse suddenly and violently.