Why are some children born with fair hair that darkens with age?

The answer to this centres around a pigment called melanin. This is a pigment that gives us dark moles, dark skin and dark hair. This is called eumelanin, and there's a slightly lighter version called phaeomelanin, which gives people red-ginger hair. Everyone has genes in their DNA that will give them different levels of these two pigments. So people who have very light and blonde hair don't have very much of these pigments at all. As your cells get older, they will change the way that they use their genes. If you have a child born with very dark hair and then becomes blond, this would suggest that the gene that's making melanin when they're a baby has been turned off. If a child is born with blond hair and becomes dark, it's because genes have started turning on melanin pigment. So it's not fixed, and even later in life you can go from being a blonder person to a darker person.