There's a difference between informed speculation and uninformed speculation and most of the opinions so far fall into uninformed speculation where the writer implicitly blames those suffering for poor diet and lifestyle choices.
In addition a lot of the speculation assumes something specific to the US where this is a trend in multiple countries, predominantly high income ones[0][1], but this speculation that it might be 'chemicals' is fairly dull to read and adds nothing. Why this cohort specifically? What commonalities are there between countries with an observed increase? If it's diet why would it only impact the younger cohort?
- insufficient fibre
- too much high fructose corn syrup
- too much milk
- too much citric acid
- toxins and parasites (gut cleanse!)
- washing chicken in chlorine (voiced as hypothetical)
- ultra-marathoners - maybe their supplements and too much carbs or dehydration?
- too much processed junk
- vitamin and mineral deficiencies
- radiation
- insufficient veggies