The world's population will officially hit seven billion on Monday, October 31 — at least according to the State of World Population 2011, a report published by the United Nations Population Fund (UNFPA). Because it's impossible to keep up with every birth and death in the world, the date reflects an estimate of reaching 7 billion rather than the exact date itself.
PopulationYear Billion
1800 1
1927 2
1960 3
1974 4
1987 5
1999 6
2011 7
Population growth did not become exponential until around 1750. Before that, high mortality counterbalanced the high fertility needed by agrarian parents. Death rates were high and life expectancy was low.
Upper Paleolithic 33 -At age 15, life expectancy an additional 39 years (total age 54).
Neolithic 20
Bronze Age and Iron Age 26
Classical Greece 28
Classical Rome 28 -At age 15, life expectancy an additional 37 years (total age 52).
Pre-Columbian North America 25-30
Medieval Islamic Caliphate 35+
Medieval Britain 30 -At age 21, life expectancy an additional 43 years (total age 64).
Early Modern Britain 25-40
Early 20th Century 31
Current world average (2010) 67.2
http://www.bbc.co.uk/news/world-15391515