How does someone's hair changes from blonde to a dark color?

Anonymous
For instance when my older cousin's son was a 1 year-old baby, he had a dirty blonde hair. But as he got older, it turned into a light browish tone.

Similar with actor Miko Hughes.
Picture of him as a child:
How does someone's hair changes from blonde to a dark color?

In the present time:
How does someone's hair changes from blonde to a dark color?


Updates
+1 y
Note: In other shows, as he was getting older... he no longer had that blonde hair such as here:
celebritykids.name/.../8362-813.gif
How does someone's hair changes from blonde to a dark color?
1 Opinion