摘要
During the last two centuries, there have been many spectacular advances in medical science, the main consequence of which has been the dramatically reduced burden of infectious diseases. While in the 1800s many people died before reaching adult- hood, nowadays most people survive. Hence average life ex- pectancy in 1800s was around 30-40, which was barely higher than it had been in Greek and Roman times （Finch, 2010）, but nowadays life expectancy in most modernised economies is around 75 - 80. This demographic shift, which has happened in only 200 years, has created a dramatic change in the causes of mortality. The major killers in the modern world are non- communicable diseases （NCDs）： principally cardiovascular disease, cancer and neurodegenerative disorders such as Alz- heimer＇s disease. A major factor that influences susceptibility to all these diseases is age. As we get older, our risk of developing these NCDs increases enormously. For example, the rate of breast cancer in females at age 15-19 is less than 10 per 100,000 population, but this increases to 100 at age 40-44, 275 at age 55--59 and 450 at age 85 ＋ （http：//www.cancerresearchuk.org/ cancer-info/cancerstats/types/breast/incidence/#age）. Ageing has consequently become a major medical, social and economic burden to many countries.
During the last two centuries, there have been many spectacular advances in medical science, the main consequence of which has been the dramatically reduced burden of infectious diseases. While in the 1800s many people died before reaching adult- hood, nowadays most people survive. Hence average life ex- pectancy in 1800s was around 30-40, which was barely higher than it had been in Greek and Roman times （Finch, 2010）, but nowadays life expectancy in most modernised economies is around 75 - 80. This demographic shift, which has happened in only 200 years, has created a dramatic change in the causes of mortality. The major killers in the modern world are non- communicable diseases （NCDs）： principally cardiovascular disease, cancer and neurodegenerative disorders such as Alz- heimer＇s disease. A major factor that influences susceptibility to all these diseases is age. As we get older, our risk of developing these NCDs increases enormously. For example, the rate of breast cancer in females at age 15-19 is less than 10 per 100,000 population, but this increases to 100 at age 40-44, 275 at age 55--59 and 450 at age 85 ＋ （http：//www.cancerresearchuk.org/ cancer-info/cancerstats/types/breast/incidence/#age）. Ageing has consequently become a major medical, social and economic burden to many countries.
