Answer:
17 Years of age.
Step-by-step explanation:
You are technically applicable for pilot's training from any age. However, you must wait until you are 14 years of age before you can operate aircraft such as gliders and balloons under any circumstance. You are freely able to operate balloons and gliders with a pilot's certificate at age 16.
However, these are simple aircrafts, and likely not the type you're implying. To answer your question, you are able to attain a pilot's license at age 17 to operate mechanically-powered-flight aircrafts, such as those that use gasoline & other fuels. This included planes, helicopters, those relatively new single-man-engine-powered-gliders that are capable of traveling great distances and reaching extraordinary heights (I forget the name), etc.