The Korean approach is still wrong, arguably. If you want to include the totality of the life of the baby, the next normal stopping point would be conception. That would make the child ~9 months old, not 1 year.
Plus, any proposal like that is inevitably going to get mixed up with the politics of abortion, which is another reason why it won't happen.
A person's Korean age starts at 1 sal, and increments at New Years. A baby could be born on December and be 2 sal by January. A baby born in January would be 1 sal until next year, and be considered 1 sal younger than the December baby, even though they were only born 1 month apart. Everyone born in the same calendar year is in the same age cohort.