The population of Seoul has dropped below 10 million for the first time in 25 years as the capital ages rapidly. Seoul's population had breached the 10-millionth mark in 1998 and once neared the 11-million mark.

According to the Seoul Metropolitan Government on Monday, the total population of the capital stood at 10.14 million as of December of last year. But the number falls to 9.99 million excluding some 150,000 residents whose addresses in Seoul cannot be verified.

As long as people have addresses in the capital on their resident identification card and are not reported dead or missing, they are counted in the official population.

The population of the capital peaked at 10.97 million in 1992, but has been declining since. The main reason is the rising cost of housing, which has prompted many to find homes in surrounding Gyeonggi Province or neighboring Incheon, according to experts. The latest census figures announced by Statistics Korea show Seoul experiencing the largest number of residents moving to other parts of the country last year. There was a net outflow of 101,000 people from Seoul in 2013, and 60 percent of those who moved out of Seoul were registered as new residents in Gyeonggi Province.

An even bigger problem for the city is its rapidly aging population. In 2013, people over 60 accounted for 16.6 percent of Seoul's population, up from 14.6 percent in 2010. The number of elderly people over 65 surpassed a million in late 2010 and is expected to exceed two million by 2027.

By contrast the number of people between 20 and 40 is shrinking. The proportion of people in their 20s fell from 15.4 percent to 14.4 percent over the same period, and in their 30s fell from 18.1 percent to 17.4 percent. The 40-something age bracket stagnated at 17 percent.

A decline in the number of economically-active people is weakening the productivity of the capital. According to Statistics Korea, Seoul's gross regional domestic product growth peaked at 15 percent in 1995, but fell to two percent in 2012. This was lower than Korea's GDP growth rate of 2.8 percent last year, demonstrating that the capital city is no longer serving as a growth engine for the country's economy.