Why are kids going through puberty earlier?

They aren't really: It may seem that way, but periods after 10 years old have been standard for over the last 30 to 40 years. The original ages everybody remembers are based on antiquated data. The higher body mass index of children may have a little effect, as might the various estrogenizing chemicals in our environment (air fresheners, bpa, etc). But the ages aren't what we remember them to be.

Difficult to tell: However, there has been various studies linking early puberty to increased weight, increased exposure to endocrine disrupting chemicals such as bpa and phthalates, and "social stress" either due to over media exposure and single parent raising. Also fetal exposure to unknown chemicals during pregnancy has been linked to early puberty. Hope this helps.