The life expectancy rate for people all around the world has increased by an average of six years compared with the rate from 1990, the World Health Organization found in its recent annual report released Thursday.

The average girl born in 2012 is expected to live to be about 73 years old while and the average boy born in the same year can expect to reach 68 years old, USA Today reported.

According to the WHO's data, life expectancy for men ranged from a high of roughly 76 years in high income countries to a low 60 years in low-income nations. Women in low-income countries are expected to live to an average age of 63 years, and 82 years for women in high-income countries.

However, there are still nine countries in sub-Saharan Africa where both men and women's life expectancies are estimated to be below 55 years.

The U.S. ranks 37th overall in life expectancy, as women's average life expectancy rate is 81 years and men's average life expectancy rate reaches 76 years, Today reported.

Japan ranked number one among its women, who are expected to live to 87 years, while Iceland's men are expected to beat out the rest of the world's male population with a life expectancy rate of 81 years.

Liberia, Ethiopia, Maldives, Cambodia, Timor-Leste and Rwanda had the best-improved rates as they all experienced double-digit gains since 1990. Liberia's life expectancy increased from 42 to 62, and Ethiopia and Maldives had a 19-year increase.

The WHO attributed the increase to a several factors and changes in the world in the last two decades.

Improvements in poor countries made to child-care and early development strategies have helped decrease child mortality rates as fewer children are dying before age 5.

Preventative efforts and better blood pressure control have allowed people in rich countries to avoid heart disease or strokes before the age of 60 and many countries have also experienced declines in tobacco use.