The reason many submissions are rejected by agents and editors is due to the writer not understanding their true audience. Take a look at these very common mistakes that often cross our desks in relation to writing for the younger audience.
1) Label your novel as targeting the age of your main character.
Young readers ‘read up’. Readers want to read about characters that are older than they are. If your main character is 14 your reader might be 8-12. Incorrectly labeling your reader market is a common mistake our agents see all of the time in submissions of this type.
Wikipedia says: The Young Adult Library Services Association (YALSA) of the American Library Association (ALA) defines a young adult (reader) as someone between the ages of twelve and eighteen. Authors and readers of young adult (YA) novels often define the category as literature traditionally written for ages ranging from sixteen years to the age of twenty-five, while Teen Fiction is written for the ages of ten to fifteen. 
Think of your widest possible audience. The protagonist is usually on the older side of your readership. That means a 12- or even 13-year-old hero for MG and a 17- or 18-year-old for YA. If you have a college age protagonist, this pushes you into the New Adult category. (In the general market this means ‘very sexy’.)
2) Write a 100,000 word literary novel for a MR audience. Generally a Middle Reader novel is between 30,000 to 50,000 words and a YA between 50,000 to 75,000 words.
While fantasy fiction can be the exception to the rule, you better write like J. K. Rowling if you are going to present a debut novel that large. And every word needs to count. Edit out all unnecessary words, and tighten the writing. This audience is a digital sound bite one. Think instant messaging and Tweeting. Write to them in shorter, no fluff sentences.
3) Forget the age of your reader and use throw back cultural language they will not get. Example: “Laverne and Shirley aren’t capping bottles in Detroit anymore, if you get my drift.” Total disconnect. And while they might know what an Elvis impersonator is, you would have to make it relevant to the reader. Perhaps they ran away, and took a bus to Vegas when an Elvis Impersonator conference was going on. What a nightmare for a child! Or the protagonist’s grandfather is raising him/her and they have to tag along when he works the stage as one. J
NOTE: There is a huge difference in interests between a 5th and 10th grader.
Many retired school teachers have a heart for the young reader BUT are two generations separated from them in culture. It will take some serious research for them to catch up enough to be relevant when writing to that audience.
4) Use a lot of slang that will quickly outdate your book. Trying too hard at being hip and cool can backfire. It is best to stick to words that never go out of style. Cool, sweet, awesome, and phenomenal might work. Hip and cool are back, so one might use them as well. A good publishing editor will know and guide you. I cannot see one or two mischosen words causing a rejection, but beware; overuse of them absolutely will.
5) Bypass the parental preferences or library gatekeepers and offer your reader inappropriate material. You know better how mature your reader is and what their emotional needs are. Go ahead and write inappropriate material.
Keep in mind the book buyer. MR’s do not generally purchase their own books. Their parents do. Over use of swear words, violence and sexuality will not fly. Good literature is not smutty.
In contrast; it is not a good thing to preach at your reader. A good story can awaken a God consciousness in a reader, and introduce a reader to biblical principles, but do not sneak in, or brow beat the reader with your doctrinal preferences. Leave that for the parent and Sunday school teacher, unless you are writing a bible study or a themed workbook. Author Jill Marie Richardson did an outstanding job of taking a cultural hot topic and relating it to our faith. In Hobbits, You, and the Spiritual World of Middle-Earth, published by Lighthouse Publishing of the Carolinas, Jill overlays the Bible with the The Hobbit and The Lord of the Rings, giving us a peek into God's world of wonder and awe. This book guides readers in discovering the deep spiritual truths found in J.R.R. Tolkien's classic characters and shows how they might apply them to their own epic tale. A great book for use by families or youth groups. NOTE: This book is upfront about its mission. Nothing sneaky about the way it approaches the subject.
TIP: Find a group of unbiased test readers of the age in your market. Prepare a list of questions for them with a YES or NO they can circle. This makes it easier for them to critique.
- Did you want to keep reading after the first two pages? YES or NO
- Did you feel like the situation was believable? YES or NO
- Was there any place you got bored while reading? YES or NO
- Where did you get bored if you did? PAGE _________ and PAGE _______
- Was the main character likable? YES or NO
- and Could you relate to his or her troubles? YES or NO
Getting this kind of feedback will help you become a better writer and nail your target audience. Adding the results from research like this into your proposal is very helpful info.
INTERESTING STAT: 40 % of Young Adult readers are adults.