If it is some sort of chemical, it was introduced around 70s (later than most plastics) and is still in widespread and increasing use. Few such targets.
BPA in plastics is polymerized, but BPA in thermal paper is free molecules, which are easier absorbed. Thermal paper didn't become widespread until the 70s, so BPA is still a possible candidate.
Or the chemical had to reach a threshold concentration in the environment and/or our bodies before having a noticeable effect, which could have taken 20 years..