The average age of menarche decreased from 16.5-years-old in 1840 to 13-years-old in 1995.This trend of decline is consistent across races in the developed world, but there are significant race differences in average age of menarche; e.g. in America, the White mean is around 13, while the Black is around 12.
Early menarche is associated with a variety of physical and psychological problems, including:
anxiety
depression
suicidal behavior
substance abuse
early pregnancy
STDs
decreased height
hypercholesterolemia
heart disease
stroke
diabetes
breast cancer.
The decreasing age of menarche can be partially attributed to a variety of environmental factors but I suspect that post-industrial dysgenics is the main driving force. Individuals with worse genetic health exhibit r-selected ("live fast die young") behaviors, such as increased sexual desire, earlier age of reproduction, and, of course, earlier age of menarche.
Sources:
Rees 1995Talma 2013Reagan 2012Lacroix 2023Sýkorová 2021