The bigger driver of earlier age of menarche is percent body fat. For example, menarche was in the very early teens in ancient Rome, but it was mid-to-late teens in medieval Europe. The average medieval European girl was malnourished by modern standards. And now, along with better availability of calories, kids also go outside and play far less than they did a generation ago, so they're less active, hence fatter.

There's might also be something in the light-exposure hypothesis, except that age of menarche tends to still be lower nearer the equator than nearer the poles. Artificial lighting, if it were the cause, would have weakened that difference.