UNITED NATIONS (Reuters) - At U.N. headquarters, a population clock ticked off the world's six billionth human among the 370,000 babies born Tuesday, many of them destined for a future of poverty and illiteracy.

In Bosnia, visiting U.N. Secretary-General Kofi Annan marked the milestone ``D6B'' by honoring a boy, born two minutes past midnight to Fatima Nevic in a Sarajevo hospital, as the world's symbolic six billionth person.

Bangladesh organized a parade; London planned a rally and China, with the world's largest population of 1.25 billion people, held an array of ceremonies.

Chinese officials began the day by contending the world's population would have hit six billion much sooner if they had not imposed a controversial one-child policy 20 years ago.

The world's population has doubled since 1960 and increased by 1 billion people in the past 12 years. But the rate, albeit distressing, is expected to slow somewhat by 2050 to perhaps, 8.9 billion people, U.N. demographers say.

In the West and in Japan, the population is aging faster than in any time in history. But in the developing world, more than 1 billion teenagers are reaching their reproductive years, the largest youth generation in history.

They live in the poorer nations of Africa, Asia and Latin America and, according to the U.N. Population Fund (UNFPA), what they do with their lives will determine the future of the world's population growth.

``There are 350 million women who would use contraceptives, if they could get them. They either don't want to get pregnant again or want to space the next pregnancy out,'' said Dr Nafis Sadik, the fund's executive director.

UNFPA and voluntary organizations take credit for promoting access to birth control over the past 30 years, giving women a choice of having fewer children.

In developing countries overall, birth rates have dropped by half since 1969, from almost six children per woman to an average of under three today.

Education Lowers Birth Rate

But the biggest impediment is still a lack of education and illiteracy, accompanied by opposition from religious leaders who oppose contraceptives or family members appalled at teenage brides marching to family planning clinics, says UNFPA.

``Surveys show that women in all regions of the world have more children than they want,'' Sadik said. ``This means that they don't have access to information or contraceptives or they aren't allowed to make decisions for themselves, an unfortunate fact in many societies.''

Three out of four African women bear their first child as teenagers and 40 percent of births are to women under 17, according to the Guttmacher Institute. The death rate is 1.5 times higher for children under 5 born to teenage mothers.

Resources, however, are scarce to make an appreciable differences in family planning over the next few years. A landmark 1994 Cairo Population Conference estimated the price of stabilizing the world population at $17 billion for universal primary education and health care, about half of which has materialized.

Consequently, U.N. demographers still foresee an annual increase of 78 million a year -- the size of the population of France, Greece and Sweden combined, or a city the size of San Francisco every three years.

The projection is a world population growing up to 10.7 billion by the year 2050 with 8.9 billion people considered the most likely.

The fastest-growing regions are sub-Sahara Africa, where the average woman has 5.5 children as well as parts of South Asia. The average woman in Latin America bears 2.7 children.

But as one world has a booming young generation, another is rapidly growing older, due to rapidly falling fertility rates well under two children per family. In North America, Europe and Japan, the United Nations estimates a startling quarter of the population will be older than 65 years of age by the year 2050.