The impending situation of too many oldies and not enough youngers walking the earth in the near future.

It is the natural outcome of several technological breakthroughs and social transformations that have occured in the last fifty years in most countries, co-inciding with a large cohort of baby boomers approaching retirement age. In many countries contraception, the weakening of the family as an institution and greater economic opportunities for single women has led to fewer partnerships and births, while improved working conditions, nutrition and health care has raised longevity. It has been identified as a potential fiscal catastrophe, with Generation X and Generation Y potentially being slugged with extra taxes to replenish the thread-bare pension funds of their parents.

Ageing can be measured through the dependency ratio (the portion of people aged 15 to 64 to everybody else), which is a composite of fertility, mortality, emigration and immigration rates. Developed countries are more likely to comprise of an ageing population than countries without an established health care system. Immigration and pro-natalist policies has allowed countries like the United States, France, Australia, Canada and Norway to assauge their dependency ratio; notably the less prepared countries are Japan, Italy, Greece and Germany.

This expression was coined by Ambassador Julia Alvarez, a member of the Permanent Mission to the United Nations from the Dominican Republic. Paul Wallace wrote about the impact ageing would have on the world in his 2001 book Agequake : Riding the Demographic Rollercoaster. And as the number of oldies rises, their political might increases to the point that they can introduce inter-generationally inequitable policies to advantage their generation. Some have coined this influence as Grey power.

However increases in the lifespan have also been accompanied by improved health amongst older people, giving policy planners to option of encouraging citizens to remain in work. Other people advocating Zero Population Growth see an ageing population as a worthwhile price for saving the planet from the ravages of over-population. And others think pensions are unlikely to shrivel up as long as aggregate productivity continues to rise and dividends are repatriated to funds. However the August 2003 heatwave in France which killed an estimated 15,000 senior citizens showed the limits of the welfare state in protecting people on the right-hand side of the dependency ratio colon.