I had dirty blonde hair as a kid/teen/young adult. Pushing 50 now and it's pretty much brown. My husband, like yours, had white hair as a kid. Not sure when it changed, he was 18 when I met him, and it was not that color anymore. It's much darker now, not sure I would say dark brown though, it's probably a bit lighter than mine.
My dd had white hair though childhood, she is almost 23 now, and it's getting darker/muddy, but still blonde. I expect it will continue to darken as she gets older.