Although the World Bank has no economic data for North Korea, all reports indicate that the level of living is very low--vastly inferior to South Korea, which has been one of the "Asian Tigers" in terms of economic development. The best evidence of this difference lies in life expectancy--estimated to be 83.3 years for females in South Korea (higher than in the United States), but only 71.8 in North Korea (about the world average). With the dramatically lower mortality, you might have expected the south to have grown considerably faster than the north. But that divergence is a fairly recent phenomenon. Until the 1990s, when South Korea emerged economically, the two countries had very similar levels of mortality.
There has also been a recent divergence in fertility trends. At the time of the Korean War in the early 1950s, South Korea had a TFR of 6.3 compared to 3.8 in North Korea--Soviet-style regimes have historically had below-average fertility levels. It was not until the 1980s that fertility in the South dropped to a level below that in the North. Since then it has continued to drop to a level well below replacement in the South (estimated now to be 1.3) compared to a drop in the North to just replacement level (2.1).
The very rapid fertility decline in South Korea created a youth bulge in the 1980s that represented a significant threat to political stability in that country, described in a now classic article by Gary Fuller and Forrest Pitts ("Youth cohorts and political unrest in South Korea,"Political Geography Quarterly, Vol. 9. No. I, January 1990, 9-22). The country survived that threat and used the youth bulge as a demographic dividend that helped create their economic miracle. The North could have done the same, but Kim Jong-il obviously was not interested in such an outcome. Will his son lead the country to a different future? The demographics are actually in favor of that, with an already low level of fertility and a level of mortality that is actually not as bad as you might think given the overall level of repression that is widely reported to exist. I don't know of anyone who expects improvement, however.