A seven to nine year old could read it if her reading skills were well in advance of most children that age. However, a child that age would probably only read it if her interests included astronomy. If she's not interested she'll never read it without massive bribery. And even if a child is extremely bright they still will usually not start to develop the capacity for adult levels of abstract thought until they become teenagers, so even if she was interested and read the book and retained a great deal of it, she might have difficulty applying what she learned to different situations to determine what is reasonable astronomy and what is bad astronomy. Remember that many seven year olds still believe in Santa Claus and other gods.