Life expectancy is a valuable metric that can be used to gauge the overall health of a population. It is the average number of years a person can expect to live assuming they continue to age at the same rate as the current population―in other words, how long a person can expect to live given current trends. It helps us to understand how long individuals will be around, as well as what health care needs they may have, which can be beneficial when setting up health care and retirement planning.

Life expectancy is calculated based on lifespan trends of the general population, which takes into account death rates, health trends, and other factors like lifestyle and living conditions. Generally, life expectancy figures are determined by national and international statistical sources, like the World Health Organization (WHO). Life expectancy can be determined for a given population, or for a specific individual depending on their age, sex, genetic makeup, and lifestyle.

When determining life expectancy, there are a few factors to take into account. Age is a key factor that can greatly impact life expectancy. Generally, the older a person is, the shorter their life expectancy. In addition to age, lifestyle choices like smoking, substance abuse, and physical exercise also play a role in determining life expectancy. Other population-level factors, such as the availability of medical care, economic stability, and poverty levels can also be taken into consideration when determining life expectancy.

Over the last few decades, life expectancy has increased dramatically in the United States and other developed countries due to improved medical care, healthier lifestyles, and better nutrition. Life expectancy has also increased in other parts of the world due to advances in public health and medical care. Globally, life expectancy is increasing, but the rate of increase is significantly slower in developing countries compared to developed countries.

In conclusion, life expectancy is a valuable metric that is used to understand a person's likely lifespan based on actuarial science and other population-level factors. And though life expectancy has risen in recent years, there is still a wide discrepancy between developed and developing countries. Therefore, it is important to remember that life expectancy is just an estimate, and that any individual-level factors or lifestyle choices can improve or worsen life expectancy.