Some details are difficult to slip seamlessly into the story, but you can show a character's age without it feeling awkward.
In some novels, age doesn't matter. The characters are adults, and whether they're thirty or fifty the book unfolds pretty much the same way. Readers can assume the characters are about "the same age as they are" (if they're adults of course) and it still tracks.
It's more about the characters being relatable to the readers' experiences than the actual age, and this also holds true for younger readers as well. As long as the characters fit the reader's expectations of age, the story flows smoothly.
But imagine a young adult (or worse—a middle grade) novel if it was suddenly revealed halfway through the story that the protagonist was thirty. The entire book would change, and everything read so far would probably feel...icky. Readers would have interpreted the entire story through the wrong perspective.








