It needn't be a knife, it could be "any other article [...] which is sharply pointed (http://www.statutelaw.gov.uk/content.aspx?LegType=All+Legislation&title=criminal+justice+act&Year=1988&searchEnacted=0&extentMatchOnly=0&confersPower=0&blanketAmendment=0&sortAlpha=0&TYPE=QS&PageNumber=1&NavFrom=0&parentActiveTextDocId=2116646&ActiveTextDocId=2116825&filesize=5719)", and the minimum age was changed to 18 (http://www.opsi.gov.uk/ACTS/acts2006/60038--e.htm#43).