`生老病死` is a formal and profound term, typically reserved for serious or philosophical contexts.
In Philosophical Conversations: It's frequently used when discussing the meaning of life, mortality, or when reflecting on major life events. A friend might say it to another after attending a funeral or visiting a sick relative.
As a Form of Consolation: It can be used to comfort someone grieving a loss, by framing death as a natural and unavoidable part of the life cycle. For example, “生老病死, 人之常情” (shēng lǎo bìng sǐ, rén zhī cháng qíng), meaning “Birth, aging, sickness, and death are the ordinary course of human life.”
In Literature and Media: It's a common motif in Chinese films, songs, and novels to evoke a sense of profundity, melancholy, or to comment on the fleeting nature of human endeavors.
Connotation: The term is generally somber and serious, but not necessarily negative. It is most often neutral, representing a calm and realistic acceptance of a fundamental truth. It is rarely, if ever, used in a lighthearted or casual manner.