The estimated population of Bangladesh (2001) was 131,269,860, making Bangladesh one of the ten most populous countries. The overall density, 890 persons per sq km (2,304 persons per sq mi) in 2001, is much higher than that of other countries except for microstates such as Singapore. Bangladesh supports a large rural population, with 21 percent of the Bangladeshi people classified as urban in 1999. The distribution of the population is relatively even, except in the sparsely populated Chattogram Hill Tracts District and the almost totally uninhabited Sundarbans. Most of the people are relatively young, nearly 60 percent being under the age of 25 and only 3 percent being 65 or older. Life expectancy at birth is 61 years.