Reading Age Range Made Simple?

I see lots of writers asking agents and editors, "What's the age range for Middle Grade? Or Young Adult?" And here's me, being of superior intelligence (when compared with a worm), guffawing to the point of nearly passing gas. Come on! It's simple!!!

Picture Books - ages 1-5
Chapter Books - ages 5-8
Middle Grade - ages 8-12
Young Adult - ages 12 and up

But WAIT! Maybe it's not that simple. Maybe these are valid questions to ask... again, and again, and again.

I was just over at Publisher Weekly's website, reading the Children's Book Reviews for the month. It's a great way to see what's coming out and if the premis to your book has been stolen, er... is already on the shelves.

http://www.publishersweekly.com/pw/reviews/childrens.html

Out of 34 book reviews... these are the age ranges called out

Ages 2–up 

Ages 3–5 

Ages 3–6 

Ages 3–7 

Ages 4–6 

Ages 4–7 

Ages 4–8 

Ages 4–9 

Ages 4–up 

Ages 5–7 

Ages 5–8 

Ages 10–up 

Ages 12–up 

Ages 14–up

Ages 15–up

15 different age ranges for 34 books! Holy Cow! No wonder there's so much confusion about age ranges. Now I'm confused. And, from this chart, it seems 9 year olds are mostly neglected.