The text of this book is pure James Herriot, with absolutely no attempt to speak down to a young audience. Therefore, the use of the regional dialect may be a bit of a challenge to some young readers, but the exquisite illustrations should spur nine year olds & older to read it for themselves. It should be stressed that this is a true story as all of Herriot's writing is based on his experiences as a country veterinarian.
Do resist the temptation to skim through the book for the illustrations before reading it though. The illustrations are definitely a part of the story and one of the double page illustrations will definitely get a better reaction if not seen until one reads the preceding text.
If this "children's book" is left out on the coffee table, I predict that everyone in the family will at one time or another read it and enjoy it. I'm sure that any teenagers will wait until nobody sees them reading it, and that they won't admit enjoying it, but chances are they will.
Also, this book may well inspire those 12 and older to read James Herriot's adult books.