I would definitely recommend this book to any age, even though it's classed as a children's book. I am 16 and found it extremely enjoyable to read.
Colin Dann allows you to almost become the cats in the book. He takes you through their rough winter in the city and allows you to feel that you are a wild cat in their situation. You become the cats that are weary of humans and risk their lives crossing the busy London roads, and you are taken through each of the cats' individual experiences and challenges.
If you love cats, you have to read this! Even if you're not too keen on cats, I still think that you'll enjoy it.