From HMRC website:
From pay reference periods starting on or after 1 October 2006 the special rules for apprentices will be extended to apprentices aged over 25. This will mean that:

apprentices under age 19 will not qualify for the national minimum wage
apprentices over age 19 and in the first 12 months of their apprenticeship will not qualify for the national minimum wage.
(Marmite Omelette ...but I feel better having screamed, don't you?, Wed 2 Jun 2010, 14:54, Reply)