人口老化是指某地某时期内总人口中老年人口比例增加的动态过程。根据联合国世界卫生组织定义，65岁以上老年人口占总人口的比例达百分之七时，称为“高龄化社会”(aging society)；达到百分之十四时称为“高龄社会”(aged society)；如果老年人口比例达到百分之二十时，则称为“超高龄社会”(hyper-aged society)。人口老化多见于发达国家，特别是欧洲国家。由于计划生育的影响，中国已于1999年进入了老龄化社会，在2004年底65岁以上人口占总人口比例为7.6%。
Among the countries currently classified by the United Nations as more developed (with a total population of 1.2 billion in 2005), the overall median age rose from 29.0 in 1950 to 37.3 in 2000, and is forecast to rise to 45.5 by 2050. The corresponding figures for the world as a whole are 23.9 in 1950, 26.8 in 2000, and 37.8 in 2050. In Japan, one of the fastest ageing countries in the world, in 1950 there were 9.3 people under 20 for every person over 65. By 2025 this ratio is forecast to be 0.59 people under 20 for every person older than 65.
Population aging arises from two (possibly related) demographic effects: increasing longevity and declining fertility. An increase in longevity raises the average age of the population by increasing the numbers of surviving older people. A decline in fertility reduces the number of babies, and as the effect continues, the numbers of younger people in general also reduce. A possible third factor is migration.
养老院：retirement home ; nursing home
A retirement home is a multi-residence housing facility intended for senior citizens. Typically each person or couple in the home has an apartment-style room or suite of rooms. Additional facilities are provided within the building, including facilities for meals, gathering, recreation, and some form of health or hospice care.The number of retirement homes has been increasing in the United States, from almost 11,000 in 2005 to 12,000 in 2010. The market size for retirement homes has had a much greater growth rate, from $13.5 billion in 2005 to $17 billion in 2010, with California being the American state with the largest market size.