I started my training at 14 (now 15, 16 next month) and have found it fantastic. I plan to do some of the various exams before my solo later this year which I have been recommended to do by various members of my flying school. Starting early has not effected me at all. I try to fly every couple of weeks and will continue to do so upto my solo where I then hope to have the lessons more regularly. In terms of it getting in the way of school exams and other studying, yes, it can add a pressure on you, but as long as you plan things out i.e not doing flying exams/excessive studying whilst major school examinations are happening it should not be a problem. I'm using this approach and SO FAR, have not had any problems regarding pressure, but I bet my answer to that would have change this time next year.
It is obviously up to you, but I would reccomend starting when your 14, as long as you think you will be able to concentrate on other activities. Good luck
