I’ve had Korean friends who don’t know exactly how old they are. Among other reasons, because of a high infant mortality in the past births weren’t even reported until long after the fact.
It’s nearly like counting a baby’s conception as the beginning from which to count their age. For the nine months of the pregnancy, they count that as the first year.
That is what I was told by some Koreans in Korea. Other Koreans may have a different slant on the origins of the practice.