It depends. I've done some reading that indicates puberty is hitting little girls earlier and earlier. It's fairly traditional for boys around 13 to have puberty onset, usually starts around 12 for them.
The theory is being tossed around that since a modern diet is so healthy, relative to what our dirt-eating ancestors had, and makes calories so abundant, our children may become able to attain full physical maturity earlier than previous generations. There's also writing that suggests modern humans are just evolving in that direction to adapt to current conditions.
A lot of girls of my generation had their periods around 12 or 13. Once you have your period, you can reproduce (though it's not necessarily a good idea). It's really not all that far-fetched to imagine a girl having her period at 10.