By Madeline Holcombe, CNN (CNN) — If you want to live a long and healthy life, you’d be better off in South Korea than the United States. In the 1980s, our country was about average in terms of life expectancy for developed nations. But since then much of the world has improved, and the United States has dropped toward the bottom of that list. On the other side of the world, life expectancy has been growing fast in South Korea, with a massive le…