Koreans are primarily from one ethnic family and speak one language. Sharing distinct physical characteristics, they are believed to be descendants of several Mongol tribes that migrated onto the Korean Peninsula from Central Asia. In the seventh century, the various states of the peninsula were unified for the first time under the Silla Kingdom (57 B.C.-A.D. 935). Such homogeneity has enabled Koreans to be relatively free from ethnic problems and to maintain a firm solidarity with one another. As of the end of 2008, Korea's total population was estimated at 48,607,000 with a density of 486 people per square kilometer. The population of North Korea is estimated to be 23,200,000. Korea saw its population grow by an annual rate of 3 percent during the 1960s, but growth slowed to 2 percent over the next decade. In 2005, the rate stood at 0.44 percent and is expected to further decline to 0.01 percent by 2020. A notable trend in Korea's demographics is that it is growing older with each passing year. Statistics show that 7.2 percent of the total population of Korea was 65 years or older in 2000, and 10.3 percent was in 2008. In the 1960s, Korea's population distribution formed a pyramid shape, with a high birth rate and relatively short life expectancy. However, age-group distribution is now shaped more like a bell because of the low birth rate and extended life expectancy. Youths (15 and younger) will make up a decreasing portion of the total, while senior citizens (65 and older) will account for some 15.7 percent of the total by the year 2020. The nation's rapid industrialization and urbanization in the 1960s and 1970s was accompanied by continuing migration of rural residents to the cities, particularly Seoul, resulting in heavily populated metropolitan areas. However, in recent years, an increasing number of Seoulites have begun moving to suburban areas.