Hair color changes with age. My mother in law and wife were both blond when they were kids, but their hair slowly turned browner with age. They both highlight their hair to split the difference.

We have two fairly young kids. Their hair is pretty light blond on the top layers, but their bottom layers are quite a bit darker. I suspect the biggest contributing factor beyond genetics is sunlight. Both of them spend a pretty good amount of time outdoors when the weather permits.