As a writer, you think of yourself as a creative person. Your sole job is to produce a phenomenal product both technically and creatively. As an author, your job goes well beyond writer. It is a marketer, publicist, and a salesperson. Once you make the decision to query agents you’ve become a salesperson, your job with your query is to sell your book to me in a way that makes me want to read more. Once I request material it’s the book’s job to do the rest.
Any good salesperson knows that you need to know your market so that you’re targeting the right customer. It doesn’t make sense to submit your adult fiction to agents who only represent children’s nonfiction. Sure, you might think it can’t hurt, and I suppose it can’t, other than being a huge waste of your time and energy. Time better spent working on your next book and targeting customers who are in the market for what you have to offer.
Before you even finish your book, do the work to understand the market. That doesn’t mean just reading genre descriptions. It means reading the books. Young adult focuses on a younger protagonist, however that doesn’t mean that all books with 17-year-old protagonists are young adult. There are a certain style and voice to young adult. Know it, because no book is both young adult and adult. No book is both fiction and nonfiction and no salesperson ever succeeded by trying to make up a market for her product.