In 2011, the average age of a first marriage for South Korean women hit 29.14, up from 24.8 in 1990; for men it jumped to 31.8 from 27.9 in 1990. The birthrate sunk to 1.15 children per woman, the lowest among the world’s most developed countries.From here.
For what it's worth, developing and undeveloped (i.e. "Third World") countries generally have lower average ages of first marriage and higher birthrates than developed countries. So, South Korea probably has the world's lowest birthrate.
I suspect that the reasons closely parallel those behind similar trends in Japan.
Japan and Korea are lands of great economic opportunity for women. But, only if they remain unmarried. Married women, and in particular, married women with children, are expected to make their marriages and raising children a priority. Their economic opportunities are reduced far more, relative to unmarried childless women, relative to comparable women in the West.
In the West, large proportions of unmarried women of childbearing age have children out of wedlock, particularly in those countries with generous social welfare systems like Sweden.
But, both Japan and Korea are much more successful at discouraging women from having children out of wedlock (despite relying very heavily on condoms relative to oral contraceptives for birth control), in part, because their social welfare system is far more tied in the case of men to a connection to a private business (an almost neo-feudal approach sometimes characterized as "Prussian") with social welfare for women and children, in turn, much more closely tied to their fathers.
Another factor that reduces the out of wedlock is that these societies are much more egalitarian. While they have large "working class" populations, they have very small "underclass" populations - so, few women are in the hopeless situations faced by women in American ghetto environments. The share of people who are "middle class" in values and in having a sense of having a viable future if they "play by the rules" is greater.
The bottom line is that in this context, far more women are deciding that they will choose a career over marriage and family, at least until a quite advanced age, than in other Western countries. And, the later you marry, the less time you have to have children and the less inclined you are to do so.
Is this sustainable?
I suspect not.
The level of angst in these countries that many women feel about being in confining marriages, or about being denied family life by careers rivals that of the American Feminist movement in the 1960s. Men and women alike are in boxes that they don't terribly like, but they aren't necessarily thrilled about the alternative model that the West offers them. Sooner or later, the dam will break and a social revolution with an uncertain outcome will break out. What alternatives end up arising out of that social revolution is hard to predict.
In the meantime, South Korea, like many developed countries, faces the risk of an inverted population pyramid that will make it harder and harder for the younger generation to support the older generation, and historical hostility to immigration in Korea and Japan make these barriers harder to overcome than they are in the United States and Europe which are relatively speaking, immigrant friendly.