Why does blond hair tend to darken with age?

If your blond preteen seems to be turning into a brunette, you're not imagining things. For some children, hair that starts out platinum blond gradually becomes darker and may end up dark brown, or even black, as they grow.

The short answer is that hair color is controlled by a complex interplay of genes, pigment-producing cells and, later in childhood, hormones. Scientists understand many of the genes involved in hair color , but they are still trying to find out why some blond children's hair darkens while others' locks remain blond for life.

"It depends on their genetic inheritance," Desmond Tobin , a professor of dermatological science at University College Dublin, told Live Science in an email.

Hair gets its color from a pigment called melanin, which is produced by specialized cells called melanocytes in hair follicles. There are two main types of melanin: eumelanin, which produces brown and black shades , and pheomelanin, which contributes yellow and red tones to hair. The amount and type of melanin produced by a follicle largely determine the color of the hair that grows from it.

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In general, more eumelanin means darker hair . For example, during pregnancy, high levels of estrogen and progesterone can encourage hair darkening as more eumelanin is produced.

The activity of hair follicles isn't necessarily fixed throughout childhood. As children grow, the biological signals controlling pigment production can change, too.

For many blond children, the most noticeable darkening happens around puberty, typically between about 10 and 13 years old, Tobin said.