NOTE: I am not a parent but can't figure out how to change my status as a parent! I wouldn't give it to a boy until around age 13 though for that reason. I believe her age would be around 14-15, but as this is shojo manga for girls in Japan, it is not what I would see as inappropriate for a girl reader in the US age 9+. There is a scene in this manga where the main character is bathing in a river nude, but it is non-sexual. There really is no manga for 6-10 year old American kids going by American standards, so if you're opening kids up to manga, I would start around age 9 youngest, and accept the fact that there will be monsters, slashing violence, and possibly non-sexual nudity in even the most benign manga you will find. There are also scenes with tertiary bad-guy characters beheaded, but this is manga, and the deaths of characters from slashing objects is pretty much in every manga other than the cringey fluffy cat manga that are for 2-5 year olds. That would be the harshest scene in the first 15 chapters.
There is one scene where a crow demon inhabits the chest of a corpse it is reanimating, but the crow is quickly defeated by a good demon and the main character. Child readers may at first find a monster scary, but then often come to learn that it was an illusion and wasn't what it seemed, often monsters even becoming good characters over time. However, there are degrees, and I would consider the spirit-type of monsters in this manga to be acceptable to as young as 9. Most manga is violent and contains monsters or scary characters, and non-sexual nudity. I have been looking for Manga for my 9 year old daughter, and due to the cultural differences in what is appropriate in Japan this requires reading every manga first (a serious pain in the rear). Full disclosure, I have only read the first 15 chapters so far, but I will definitely finish it.