On the eve of the founding of the People's Republic of China, the average life expectancy of Chinese people was 35 years, while in the 21st century, the average life expectancy of Chinese people has exceeded 70 years. In the early 20th century, 50 years was once regarded as the standard for old age. With the improvement of living standards and medical conditions, the age standard for "old people" has been constantly postponed. According to the World Health Organization, people aged 55 to 64 are middle-aged, those aged 65 to 74 are young elderly, and those aged 75 to 89 are considered elderly. People aged 90 to 120 are very elderly.
So, how long can humans live? In biology, there is a calculation standard for the "physiological lifespan" of mammals: the length of the physiological lifespan should be 5 to 7 times the growth period. For example, dogs have a growth period of 2 years, and their lifespan is 10 to 14 years; cows have a growth period of 4 years, and their lifespan is 20 to 28 years; horses have a growth period of 5 years, and their lifespan is 25 to 35 years. The growth period of humans is calculated by the time when the last tooth grows out. Most people grow their last tooth between the ages of 20 and 25, so the shortest lifespan of humans is 100 years, and the longest is 175 years.
In addition, there is another calculation method. Every day, many cells in the human body age and die. For example, an adult weighing 70 kg loses about 2×10^11 cells every day, which need to be replaced in time. In the human body, the number of cells that can divide and proliferate is limited, with an average of 50 divisions. This phenomenon was discovered by American biologist Hayflick in 1961, and this cell division limit is now known as the "Hayflick limit". Hayflick believed that there are many cells in the human body that can divide and proliferate, and these proliferating cells can divide every 2.4 years to replace the dead cells. Therefore, as long as the cells in the human body can be constantly renewed, humans can live. Based on the number of divisions and the interval time of proliferating cells in the human body, he estimated that the normal lifespan of humans should be 50×2.4 years = 120 years.