Not exactly what you're going for, but if I remember right, I've read in some anthropology/evolutionary psych literature that girls who grow up in poorer/harsher life circumstances tend to hit menarche earlier than those in more comfortable surroundings. There might also be some correlation with ethnicity, but that can be hard to sort out from the kind of environmental factors I first mentioned. The Ev. Psych argument goes something like "the bodies of girls in less comfortable circumstances pursue a strategy of something like "get some kids out while the gettings good!", whereas the bodies of girls in more comfortable circumstances recognize the opportunity to be choosier, which might mean waiting longer to make babies.

Of course, all of this assumes that a) you think Evolutionary Psychology is a valid/useful field, and b) that any subtle effects are utterly dependent on physiological ones. It also would go against the general trend you're proposing if things are basically "better" physically for most girls, but that might also make for useful triangulation: if it's historically true that girls in poorer/more severe circumstances hit menarche earlier than more comfortable girls, but these days, even the comfortable girls are hitting menarche earlier, that might be a very telling data point.

Thanks,
Jeff