Much akin to the advice do not say that your target audience is everyone, do not include in your query or pitch that your novel is a crossover. Crossover novels are a cultural phenomenon that cannot be predicted (at the very least, your agent or editor will decide to target a wider audience if applicable).
In case you don't know what a crossover novel is, it's a novel that begins either as an adult novel but appeals to young adults, or starts as a young adult novel but appeals to adults. Lord of the Rings (adult to young adults), Harry Potter (middle grade to young adult to adults), The Hunger Games (young adults to adults), are only a few.
To me, there is no such thing as crossovers. I see so many adults reading YA that it's becoming a pointless term, and of course young adults have always read adult novels. But we do need our labels and it makes organizing a library much easier.
Tell me dear readers, your favorite crossover novels? Do you aspire to write the next great crossover?