It's common for most European people I think.

In France, "têtes blondes" (meaning "blonde heads") means children in general, because most children are born with blonde hair, even though they turn dark later on for most of them. Although this is about hair, eye colour is very much correlated with this.

I'm pretty sure that's only the case for people of European descent though, bar probably a few exceptions.

Children of African descent do sometimes, perhaps often, have blue eyes when very young. A remember a co-worker teased about her child's "contacts". (Let me be clear: teased by other women of color.)

In the US (south?) they're called towheads.

In Germany there's a common folk wisdom that "all babies are born blue-eyed". Of course, as the article points out, the exception is when they're born brown-eyed.

Eastern European here. I was blonde as a toddler and over time my hair turned brown. Eyes on the other hand were deep dark brown in childhood but washed out to a much lighter shade of brown after adolescence.

Same in England, granted there's probably shared genetics in there somewhere. I was bleached blond and had bright blue eyes. Both are now about as dark a brown as you could get.

Not Swedish, but blue-eyed, like my father and maternal grandfather, and I was blonde (like my father, and maternal grandfather) until early puberty, when my hair turned brown and wavy. My mother's hair was jet black.

So it can be more than four years, in my case it started when I was eleven years old.

Happened to me, was blonde when I was a kid, now dark brown. My eyes shifted a little but not too much, from blue to blue-gray.

My parents preserved some of my baby hair in a matchbox. Apparently I was blond like a Lannister at the beginning, even blonder maybe. I still have the blue eyes but I will be considered blond only in the Middle East. Most of the darkening happened before I start school but definitely continued until adulthood.