A lot of people with curly hair find that it straightens with age. That happened to me. As a child I had Shirley Temple hair. Then it gradually loosened. It's still wavy, though.
Red haired people often similarly find that the hair turns brownish in middle age, or at least that "vibrant" red in the hair dulls. That happened to all my red-headed aunts and cousins.
As you said, blonde children often become light brown haired or even medium brown haired as adults. In fact, from my experience, I think that it's more common than not that Europeans as a whole are lighter haired as children than they are as adults.
It must have something to do with the interplay of hormones with the pigmentation snps.
I think severe physical or emotional shocks can also change hair color. Barbara Bush often said that when her young daughter died of leukemia her hair turned white over the course of a few days and she kept it that way in remembrance of her. In my own family, the same thing happened to my grandmother.
The only change as severe as yours of which I'm aware was with my own son. He was born with a full, and I mean "full" head of black hair. My husband was over the moon, thinking he would look like him.
Over the course of the next few months it turned platinum blonde, and none of it fell out, which as you might know usually happens to the hair of a newborn. He had his first haircut at around 9 months.
It stayed blonde, and straight, until high school when it turned a medium brown. In the summer it still bleaches a bit, and if long it has a bit of a wave.
Moesan didn't believe me. I had to post the pictures to prove it.