Data on the prison population, including a breakdown by age, are published monthly on the Ministry of Justice website:
http://www.justice.gov.uk/publications/populationincustody.htm
The following table shows the number of young people held in prison establishments as at the end of each month since January 2007:
15 to 17-year-olds Young adults1 January 2,385 9,276 February 2,418 9,286 March 2,413 9,311 April 2,402 9,425 May 2,450 9,364 June 2,428 9,455 July 2,472 9,418 August 2,473 9,545 September 2,485 9,533 October 2,441 9,544 November 2,357 9,542 December not yet available 1 Young adults are those aged 18 to 20 and those 21-year-olds who were aged 20 or under at conviction who have not yet been reclassified as part of the adult population.
These figures have been drawn from administrative IT systems, which, as with any large scale recording system, are subject to possible errors with data entry and processing.