When a person chooses to leave the workforce. The concept of full retirement – being able to permanently leave the workforce in old age – is relatively new, and for the most part only culturally-widespread in first-world countries. Many developed countries have some type of national pension or benefits system (i.e. the United States' Social Security system) to help supplement retirees' incomes.
Dramatic advances in healthcare have extended the lives of people in, predominantly, first-world and developed countries. That means that without adequate personal savings and/or pensions, people could easily outlive their retirement funds. In times of economic downturn retirees may choose to "come out of retirement" and re-enter the workforce on a seasonal, part-time or full-time basis to earn income and obtain benefits, especially costly health insurance coverage.