I’m guessing that much of that 19% is made up of kids. A lot of blonde kids have brown hair by the time they are adults. I have a hard time believing that 19% of Americans are natural blondes. In North Dakota maybe. In the states I’ve lived in for the past 11 years, naturally blonde adults appear to be in the extreme minority.
My hair is still getting darker. Hopefully by the time I'm 80 it will be jet black. Up till I was 19 mine was pure white platinum blonde. I liked girls whose hair was the exact oposite.