Voice
Every writer knows when they're writing there's a way of writing that couldn't be written any other way.
But it's difficult to write like this and often takes years of writing like this to know when one's writing like this or isn't.
When a writer writes like this the writing doesn't grow old for either the writer or the reader. The writing moves forward like time moves forward, sure of itself in the same way time is sure of itself.
Anyone writing like this writes like they're writing in their own voice, which is not an easy thing to do and often takes a lifetime, as writing with a voice that is sure of itself is often as fun to read as it must have been to write.