There are several cards which comply with the government-approved 'PASS' standard for ID cards. Some of them are specifically designed to prove that the holder is over-16 or over-18 but CitizenCard (which is one of the most well-known card issuers) provides age-related ID cards to all age groups:
http://www.citizencard.com/
Chris
(PS: When I was a youngster, the age for paying full fare on our local buses was 14. However, I was so determined to look 'grown up' that I insisted on paying full fare from the age of 12!).