I'm currently writing a book that goes much, much more than the average word count. (My children's novel is at least 80k words). My targeted audience would be 8-13 years old.
Will an agent reject a well-written story with a unique concept just because it's too long?
They shouldn't.
If your story is high-quality, entertaining and unique, it won't matter how long it is because the children reading it would want to continue reading it, and they wouldn't care how long it is because they would be enjoying it.
And if your agent rejects your novel solely based on word count, my recommendation is just to get a new agent, preferably one who sees the value in your work and cares about the quality of your work rather than the quantity.
Not universally. Quite regularly, first novels significantly longer than average are published. But the further away from "normal" your book is, the higher the risk to the publishers (especially with being longer, because that also increases production costs), so the better the book has to be for them to be willing to take a bet on it.
It depends.
The more your word count differs from the 'average' for that genre the more likely that you will have problems selling it.
The farther away you are from their target (which is often a range not an exact number) the worse your chances.
So if you are going to deviate you had better have a much better plot and a much better written novel. Especially if you are previously unpublished.
And when they do list a range for wordcount you should be in that range and not higher/lower.
There is a reason for word counts by agents and that is because they have to sell your mss to a publisher. And they know what makes it most likely to make the sale for you.
So to answer your question in the OP, yes, if your mss is much much more than the typical book you have pretty much ensured that no agent will take that mss and no publisher will buy it.
Yes
Publishing within a certain word count is essentially the publishers setting up a hurdle to see if you are capable of following the rules that someone gives you. As a new writer, you are an unknown. The onus of proof is on you to demonstrate that you are a good writer, and that means metaphorically coloring within the lines of the coloring book until you have proven your trustworthiness. A lot of authors have great ideas before they publish their first work that have been vindicated by history, but they had to pitch absolutely trite, garbage ideas to a publisher to get the publishing clout necessary to publish their big experiment.
If you are a well-established author with a good agent and a good track record with your publisher, they will be more likely to make exceptions or bend the rules for you. Not if you are an unknown writer.
Think of it this way. Someone asks you to sum something up in five minutes. If you fail to do so and continue talking after the five minutes have passed, even if the person tries to leave the conversation and walk away, is that appropriate?
