first period earlier on average than they did in the 1950s and '60s
That is a extremely narrow scope, quite possibly interfering with heavy industrialization as stated before. The historical data therein are not really discussed; but in such narrow scope it is true that the age of menarche has gone down - its just that there is more to it.
https://www.npr.org/2024/06/02/nx-s1-4986075/girls-in-the-u-s-are-getting-their-period-earlier-heres-what-parents-should-know
Europe too, iirc.
That is a extremely narrow scope, quite possibly interfering with heavy industrialization as stated before. The historical data therein are not really discussed; but in such narrow scope it is true that the age of menarche has gone down - its just that there is more to it.