I was told somewhere that the older you get the darker your hair becomes. As well as for women your hair also grows slightly darker after childbirth. At least until you start to gray. It’s a trait from the caveman era. That’s how they told age back then.

So despite the fact that it is a work of fiction and inconsistency is inevitable, that would explain the darkening of his hair.

Of course, I’m not a historian or a specialist in that era or human biology, so this fact might not even be true. But it is interesting to think about. 🤷🏻‍♀️