I began flight lessons when I was 15, about six months away from my 16th birthday. This time frame was absolutely perfect: I completed all the pre-solo requirements and training just as I turned 16 and soloed the following weekend.
Of course, I finished all the private pilot requirements within another six months (and thus, six months shy of the 17th birthday). However, this was no problem, since I intended to add an instrument rating to my license. My instructor allowed me to complete much of the dual instrument before the private checkride, and I finished building the necessary PIC and cross-country time for an instrument rating after receiving my private license.
I guess the time to start really depends on your future flying aspirations as well as funding. My suggestion: find a good instructor who will talk with you and your parents about flight training and layout a plan that is tailored to fit your desires. Then he/she will not only understand your immediate goals but will also be able to prepare you for future challenges and training requirements.
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"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been and there you will always long to return." --- Leonardo da Vinci