Female obesity is known to lead to preganancy complications which cause cognitive deficits.
Obesity in general is known to cause systemic inflammation which affects cognition and speeds up dementia.
The opioid crisis and various other types of drug abuse lead to brain damage.
Various nutritional deficiencies could play a role. Historically, iodine deficiency caused the condition of cretinism, but sub-clinical deficiencies can lead to more subtle effects.
Sleep deprivation is known to cause cognitive deficits, and electronic lights and jobs with nocturnal schedules are more common now. Light pollution and spending too much time indoors during the day throws off our circadian rhythms.
We still have various types of neurotoxins plaguing our environments. More people are living in urban environments now, which causes greater exposure to automobile exhaust. Coal plants are still spewing neurotoxins. Lead pipes are still common in many cities.
High indoor CO2, from poor ventilation, is known to cause cognitive deficits.