[quote=@Tatsua Aiisen]
This might blow your mind a little, but Katana are kind of hard to use. Not to mention, they're heavy, so a girl that age would probably have quite a bit of trouble keeping her balance, not to mention accurately swinging one.
[/quote]

Well, not that heavy really. A katana is generally 2.8 pounds with a length of 60-73 centimeters in length. That is to say that a blade which is 2.3 feet long is weighs only 2.8 pounds while the normal longsword, also wielded in two hand style like a katana, can weigh 2.4 to 4 lbs and have a length of 4.2 feet. The difficulty is not really with weight or length but with how it is wielded. A katana is a two handed blade with a curve to it. Thrusting with a katana is nearly impossible but it deflect blows rather easily. But, the katana can also easily be deflected by a skilled warrior.

A major issue I am currently having here is that at nine, she would have been training for 4 to 2 years before killing this man. That is if she began her training around the same time a samurai would begin training. It takes a long time for a samurai to master their blade so doing so in such a small period of time should be impossible. Heck, even a teenage samurai would find it difficult to deflect the force of a blow. As for making the age twelve, that would be fine so long as the man isn't a trained soldier.