I don't believe there is a minimum age for commencing instruction, but I may be wrong. The minimum for solo is certainly 16, and the only requirement for that is to have a valid medical certificate (and be under the guidance of a qualified instructor). If you can find it, all the information is on the CAA website, but it's not very user fiendly (I was going to change the miss-spelling on that, but the word is strangely appropiate.........)
The Cambridge Aero Club has succesfully taught teenagers (and many others) to fly for some years - give 'em a call on 01223 373214.
A