This is the Age Structure (KS102) from the 2011 Census. The data provides the number of people usually resident in an area by age group. It also includes the mean and median age.
A usual resident is defined as someone who spent most of their time at a specific address.
The 2011 census in England and Wales took place on 27th March 2011. It provides a snapshot of the population and its characteristics on that day.
Further information about the census estimates, including details about the methodology is available at http://www.ons.gov.uk/ons/guide-method/census/2011/index.html