This is a question I have had too. In the reading I've done on it, it seems that whether it goes away, stays the same or gets worse depends on which gene is causing it. And there are multiple genes that can cause it.
One is vienna(v), another is silvering(si); apparently steel(ES) can cause it too; and then there are the two spotting or broken genes, dutch(du) and english(En).
There are some modifier genes that affect how much develops, they are the genes that make the difference in silvering between silver fox and the various argent breeds. These modifiers can be inherited totally separate from the main genes.
There are two genes that cause temporary white in juveniles. One, 'snowball', causes white undercoat that goes away; the other, 'frosty', causes white guard hairs that go away. They both disappear when the fully senior coat comes in. I get snowball a lot in my herd, they go through a phase where they look like sallanders!
My senior animals slowly develop scattered white hairs over time. There are none in their first senior coat, then a few the next molt, then a few more. It's really annoying. Since I have beverens I'm guessing the source is vienna, which is present in all my bloodlines from three to six generations ago. I guess I'll never be rid of it. :thinking: