I don't have a primary source on why, which is why I didn't specify one. People have said the water one forever but I think that's only part of the case, with things like the chemicals in the food and environmental pressures also likely helping it along.

And tbh, I don't think anyone is going to really talk about it much because being the guy talking about 9 year old girls having boobs and a sexual appetite is an extremely bad look even if its correct and you are treating it like a major problem to prevent/solve.