Best for ages
Format: # up.
Example: 10 up
The age you put should be the youngest age child you think would understand and enjoy this book if it was read aloud. If you like, you can differentiate Read Aloud and Read Alone ages, this way:
Read Aloud: 10 up Read Alone: 11 up.
This, of course, is very subjective, and not really worth a lot of arguing over. Just make your best estimate.