> Lead?

Unclear. The cohort most exposed to atmospheric lead was born between 1951 and 1980 [1]. This study only measures those born before 1948.

(To the extent the study supports a hypothesis, it's the null. Given atmospheric lead increased from 1890 onwards, until about the 1980s [2], if lead were the culprit we'd expect to see more dementia among the study's younger generations. Not less.)

[1] https://www.pnas.org/doi/full/10.1073/pnas.2118631119

[2] https://www.sciencedirect.com/science/article/abs/pii/S10406...