Walk back a step with me djf881:
This is only my opinion, and Jim has been active on QLH consistently for years, and has probably seen thousands of letters and dozens or hundreds of successful ones. However, personally, I don't think you need to explain anything about who your audience is beyond the genre of your book when you are querying fiction.
Even if the agency's guidelines state: 'tell us the book's target audience?' There are some out there.
Also, I am vexed when an agency wants to know '. . .all about yourself, your publishing history, what makes you qualified to write this story,' etc. They want to know that I'm a boiler engineer? Published works: 0. What makes me qualified to write a murder mystery/ghost story/doomsday thriller with a nearly all-female cast? That's just how the story came out.
btw, you guys are the incredible best for being so generous with all your info and experience. Every bit helps.
But then, you're aware of that.
Agencies that represent different kinds of books don't always have different query guidelines for fiction and nonfiction.
The author's platform and credentials matter for nonfiction much more than they matter for fiction.
For certain kinds of nonfiction, like business texts and motivational memoirs, the author's platform as a speaker is a huge component of the publisher's decision to put the book out, since the plan is to sell the book at the author's speeches.
For other kinds of nonfiction, your background matters because it gives you the credibility to talk about the subject. For example, if you are pitching a diet book, it's helpful to be a nutritionist or a doctor.
Platform doesn't matter for fiction. You are pitching a complete work, and the work can be judged on its own merits. Your background can help; if you're writing medical thrillers, it's good to be a doctor.
But you can also write a medical thriller with no medical credentials, and if an agent who represents medical thrillers thinks the book is persuasive enough to sell to her editorial contacts, your credentials don't matter.
My novel, DON'T EVER GET OLD is about an 87 year-old retired policeman who goes hunting for a Nazi fugitive. His age-related medical issues are a big part of the novel; he has early-stage dementia and he's on blood-thinning medication. One of the key dramatic scenes of the novel occurs when a doctor confronts the protagonist with the idea that he needs to be in an assisted-living facility, because his health problems make it unsafe for him to live on his own.
I am a 31 year-old lawyer. I have no experience being 87 years old, and I am not an expert in the medical problems of the elderly. I took a couple of law school classes in criminal procedure, but I have never done any criminal defense or prosecution, and I am not an expert on crime or cops. I've got no credentials to write about fugitive Nazis.
I did a lot of online research and I spoke to a couple of doctors who helped me make sure the medical information was close to accurate. Plenty of writers without police backgrounds write about cops. Plenty of non-lawyers write legal thrillers. People with no credentials related to education or children write books for kids. Non-vampires write paranormal romances. The work is all that matters.
That said, if the query guidelines ask about audience or marketing, a one sentence response to that doesn't hurt.