system
Given the following post, respond as if you're commenting on Reddit:
human
Why are some people blonde when they're young and their hair gets darker with age?

Do we know the physiological mechanisms/inheritance? Does all hair get darker, but it's more obvious if you start blonde?
gpt
People with genes that code predominantly for brown eumelanin are the ones whose hair is most likely to darken with age. This is because although they have the genes coding for brown eumelanin production from birth, these genes are not expressed to their full potential in early life.