The Koreans are 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 2000, Korea's total population was estimated at 47,275,000, with a density of 476 people per square kilometer. The population of North Korea is estimated to be 22,175,000. Korea saw its population grow by an annual rate of 3 percent during the 1960s, but this trend slowed to 2 percent over the next decade. Today, the growth rate stands at 0.89 percent, and is expected to further decline to 0 percent in 2028. A notable trend in Korea's demographics is that it is growing older with each passing year. Statistics show that 7.0 percent of the total population of Korea was 65 years or older in 1999, while this generation made up 7.1 percent of the total in 2000. In the 1960s, Korea's population distribution formed a pyramid shape, with a high birth rate and relatively short life expectancy. However, the structure is now shaped more like a bell with a low birth rate and extended life expectancy. The young population (under the age of 15 years) will make up a decreasing portion of the total, while senior citizens (over 65 years) will account for some 19.3 percent of the total by the year 2030. The nation's rapid industrialization and urbanization in the 1960s and 1970s has been accompanied by a continuing migration of rural residents into the cities, particularly Seoul, resulting in heavily populated metropolitan areas. However, in recent years, an increasing number of people have begun moving to suburban areas of Seoul.