Leon,
I dont believe that there are specific laws covering the age required to carry a knife. So you must adhere to the same laws as the rest of us. However clearly individual places (your school for example) may forbid any type of knife - UK legal or not (and quite right too - why carry a knife to school?).
The best guys to ask round here are Martyn or Chris (Claycomb). Better yet, ask Ross over on BB (he's a policeman so he should know if anyone does - plus he is a a sound bloke so I trust his judgement)
Red