Who says you can't teach an old dog new tricks? I've always struggled with how to determine the target age of a book. Not any more. You can find a code in the front or back of a book that looks like 008-012 for ages 8 to 12 or 07-011 for ages 7-11.
Also if you are reading a book and thinking, "I wonder how many words this book has?" You can divide the number of pages by 4. So for exmaple I am reading HOOT,by Carl Hiaasen and it has 292 pages. Divide that by 4 and you get 73 add 3 zero's. (1000 words per 4 pages.)
So if you are researching to see how long your book should be to reach a ceratin target age, go to the library or bookstore and start multiplying and dividing and see if you can find the code for the target age.