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:

In the present time:

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

[somebodysaycheese]

My moms was strawberry blonde all the way to her late 20's but now is a dirty blonde. Just like skin color can darken with age so does eye color and hair color.