"The CRB reveals all convictions, cautions, reprimands and warnings that are held on the Police National Computer (PNC)."
It doesn't mention anything about being under a certain age
"On the 19th October 2009, a judgement by the Court of Appeal supported retention of criminal convictions on the Police National Computer for policing purposes. In particular the police are obliged, under Part V of the Police Act, to provide the Criminal Records Bureau with access to all convictions held on the PNC. Therefore, the stepping down of convictions is no longer undertaken. In effect this means that all convictions will be retained on the PNC until an individuals 100th birth date."
Have a read for yuorself
http://www.crb.homeoffice.gov.uk/default.aspx