What's the problem with an ageing population,specifically in china?

The older a population you have, then it means there won’t be too much people working to run and support the country in the future.

While population aging represents, in one sense, a success story for mankind (massive survival to old ages has become possible), it also poses profound challenges to public institutions that must adapt to a changing age structure. The first challenge is associated with dramatic increase in the older retired population relative to the shrinking population of working ages, which creates social and political pressures on social support systems. In most developed countries, rapid population aging places a strong pressure on social security programs. For example, the U.S. social security system may face a profound crisis if no radical modifications are enacted. Cuts in benefits, tax increases, massive borrowing, lower cost-of-living adjustments, later retirement ages, or a combination of these elements are now discussed as the possible painful policies, which may become necessary in order to sustain the pay-as-you-go public retirement programs such as Medicare and Social Security. Population aging is also a great challenge for the health care systems. As nations age, the prevalence of disability, frailty, and chronic diseases (Alzheimer’s disease, cancer, cardiovascular and cerebrovascular diseases, etc.) is expected to increase dramatically. Some experts raise concerns that the mankind may become a “global nursing home” (Eberstadt, 1997). The aging of the population is indeed a global phenomenon that requires international coordination of national and local actions. The United Nations and other international organizations developed recommendations intended to mitigate the adverse consequences of population aging. These recommendations include reorganization of social security systems, changes in labor, immigration and family policies, promotion active and healthy life styles, and more cooperation between the governments in resolving socioeconomic and political problems posed by population aging. On the positive side, the health status of older people of a given age is improving over time now, because more recent generations have a lower disease load. Older people can live vigorous and active lives until a much later age than in the past and if they’re encouraged to be productive, they can be economic contributors as well. Also the possibility should not be excluded that current intensive biomedical anti-aging studies may help to extend the healthy and productive period of human life in the future (de Grey et al., 2002).

http://longevity-science.org/Population_Aging.htm