I wonder if it has something to do with a cultural perception of how life and the world in general is perceived. As in, "He's currently experiencing his first year of life" vs. "He has lived for one year", kind of "the journey" vs. "the destination". Seeing the present more than looking back the past. In the same line of thinking as "What you're doing" vs. "what you've done", or maybe even "making progress towards goals" rather than "accomplishments".

Seems like a healthier and maybe even more productive way to see the world. I don't know enough about Korea to say whether that has anything at all to do with the way they count ages, but I feel like it's an interesting thought regardless.