The world population is growing, and it's also growing older. Researchers say birth and death rates are gradually falling worldwide, increasing the number and overall proportion of older people. For many societies, caring for all those aging citizens could be difficult.

The United Nations estimates that about one out of every 10 people on the planet today is at least 60 years old. By 2050, it's projected to be one out of five. Which means that not only will there be more old people, but there will be relatively fewer young people to support them.

Demographer Richard Lee of the Universiy of California at Berkley says this aging of the world has a significant impact on economies. "Population aging increases the concentration of population in the older ages and therefore it is costly," he said.

Aging populations consume more and produce less. With more people living longer, it could get expensive. But Mr. Lee says with continuing increases in worker productivity and smart planning, it can be manageable.

Societies have different methods for caring for the elderly, but each carries a cost. Generally, there are three types of support. Seniors can live off the wealth they accumulated when they were younger. They can rely on their family to take care of them, or they can rely on the government.

In industrialized nations, governments created publicly-funded support systems. These worked relatively well until recent years, when aging population growth in places like the United States and western Europe began to undermine the systems finances. These nations now face some tough choices. Mr. Lee says the elderly in some of these countries must either receive less money, retire later or increase taxes to make the system sustainable.

Most developing nations have not built this type of government-funded support, but have instead relied on families to care for their elderly. These nations also generally have much younger populations, which means their situation is not as urgent as more developed nations. But Mr. Lee says that doesn't mean they can ignore the issue.

"Third world countries should give very careful thought to this process, to population aging and how it may affect their economies - now, before population aging may have even appeared on their radar screens as an issue," he explained.

Mr. Lee says some governments see family support systems as a way to avoid the financial problems that industrial nations face with their publicly-funded safety nets. But he says history has shown that familial support systems are generally weakened when nations become more prosperous -- as fewer children live with and care for their parents. And he says if the nations wait too long, it may be too late

"Once population aging has become visible demographically, it's really too late to tell people who are just about to retire that they should have been saving since the time they were 20," added Mr. Lee.

For those people planning on retiring in 2050, the time to start saving was five years ago.