You must decide whether a young person is able to understand the nature, purpose and possible consequences of whatever action you propose to take. But you must also assess whether they can understand the consequences of you not taking the proposed action. For you to consider them capable, the young person must be able to understand, retain, use or weigh the information, and communicate their decision to others. The ability to consent depends more upon ability to understand and weigh up options than on age. It is important to assess maturity and understanding on an individual basis, with regard to the complexity and importance of the specific decision to be made. A young person who has the capacity to consent to simple things may not have the capacity to consent to something more complex.
Regulations for health staff at http://www.gmc-uk.org/guidance/ethical_guidance/children_guidance_24_26_assessing_capacity.asp