July 11th is World Population Day
United Nations Population Division produces regular demographic estimates and population projects for countries worldwide. Its website provides free access to reports and data including world fertility, mortality and World Population Reports. See also the United Nations Population Fund (UNPF).
UN Demographic Yearbook
Free access to all editions from 1948 onwards. Covers population size, births, deaths, marriage, divorce for nations worldwide. Also covers foreign born populations.
For UK data try the ONS website
There is also a pyramid by age and ethnicity for your local area.
For historical sources try Histpop – Online Historical Population Reports.