I don't know what planet you are on, but this has been discussed endlessly. It is because of hormones being used in meat production.
I'm afraid there are some inconvenient facts with that theory, not least of which is that it fails to explain why the trend began over a century before those hormones saw widespread use.
The average age for menarche (a girl's first menstruation) has been declining by about 4 months every decade since at least the mid-1800s. That global trend remains the case today, with better nutrition and the rise in childhood obesity being the leading theories. Given that we didn't have the technology to manufacture rBST/rBGH until the 1980s (the BST/BGH hormones had to be harvested from carcasses prior to that, which meant they saw barely any use), they obviously can't explain a trend that began ~125 years earlier.
It's also worth noting that rBST/rBGH are broken down in the course of normal human digestion, so even though they were initially implicated when people began to take note of this issue in the 1990s, they're generally believed to actually have no impact.
But let's say that's wrong. Let's say they are the cause. rBST/rBGH were banned early in the '90s in large parts of the world, so if they are to blame, we'd see a huge divide between countries where those hormones are banned (e.g. EU) and countries where they're still allowed (e.g. US). Instead, the chart I linked at the top shows quite clearly that countries in the EU like Germany are just as (or more) affected by this trend as the US is.
For the sake of argument, we might consider as well whether some other hormone might be to blame. If that were the case, we'd expect meat eaters to see earlier menarche while vegetarians/vegans remain stable. Thus, in countries like India, where nearly half the population doesn't eat meat, we'd expect the lack of meat eating to have a dampening effect on this trend. Instead, however, India has seen the average age decrease by 2 years in just the last few decades, which is an even sharper rate of decline than we've seen in the US. Moreover, if your theory was true, that would mean that the meat eaters must've actually seen a 4 year decline to drag the country-wide average down by 2 years, which would be such an enormous gulf between meat eaters and those abstaining from meat that it couldn't go unnoticed. And yet, no one is talking about it...because it didn't happen.
To be fair, there's evidence tying the consumption of red meat to earlier menarche, but the effect is far more muted (i.e. a few months, not 4 years earlier) and this seems to come back to nutrition or other causes, rather than the hormones used in meat production. Sure, you can find rags like the Huffington Post asserting as fact that the hormones used in meat production are the cause, but that theory simply doesn't fit the available evidence.
