Over the past two decades, life expectancy in the world's poorest countries has come up by an average of nine years, with the major improvements in infant health, the UN said on Thursday.
In its annual statistics, the UN's World Health Organization (WHO) said that six of the countries had even managed to raise life expectancy to over 10 years between 1990 and 2012.
The top achiever was Liberia, where average lifespans increased by a full 20 years, from 42 to 62.
"An important reason why global life expectancy has improved so much is that fewer children are dying before their fifth birthday," WHO chief Margaret Chan said in a statement.
Globally, average life expectancy rose by six years during the same period.
Based on global averages, a girl who was born in 2012 can expect to live to around 73 years, and a boy to the age of 68, the WHO said.
"But there is still a major rich-poor divide: people in high-income countries continue to have a much better chance of living longer than people in low-income countries," Chan said.
A boy born in 2012 in a high-income country can expect to live to the age of around 76 ?- 16 years longer than a boy born in a low-income country.
For girls, the difference is even wider, with those in high-income countries likely to live to the age of 82 and those in poor nations to 63.
Female life expectancy in all the top 10 countries of the globe is 84 years or more, the WHO said.
Women in Japan enjoy the world's best life expectancy, at 87 years, followed by Spain, Switzerland and Singapore on 85.1 years each.
Life expectancy among men, meanwhile, is 80 years or more in nine countries, with the longest in Iceland (80.2), Switzerland (80.7) and Australia (80.5).
"In high-income countries, much of the gain in life expectancy is due to success in tackling noncommunicable diseases," said Ties Boerma, head of the WHO statistics division.
"Fewer men and women are dying before they get to their 60th birthday from heart disease and stroke. Richer countries have become better at monitoring and managing high blood pressure for example," he added.
Declining tobacco use is also a key factor in helping people live longer in several countries, the WHO said.
At the other end of the scale, life expectancy for both men and women is still less than 55 in nine sub-Saharan African countries: Angola, Central African Republic, Chad, Democratic Republic of Congo, Ivory Coast, Lesotho, Mozambique, Nigeria and Sierra Leone.