In the 1960's, those born as females were hitting normally reaching puberty at around age 11. By 1997 that had shifted to 10 years old, and now puberty is defined as normal if it occurs between ages 8 & 13. Early onset puberty is potentially being driven by stress, increase in endocrine disrupting chemicals, body-mass index(BMI). According to recent research, poor sleeping habits also contribute to this due to less melatonin which comes from a massive increase in screen time!

Article here: https://www.newyorker.com/science/annals-of-medicine/why-more-and-more-girls-are-hitting-puberty-early