I have wondered this for quite some time, but what is the youngest age to fly any sort of plane in the USA? Does it also vary on what type of plane you're flying? (Commercial, Private, Training.)
The easiest way to figure this out for the US is to look at 14 CFR Part 61
§61.83 Eligibility requirements for student pilots. To be eligible for a student pilot certificate, an applicant must:
(a) Be at least 16 years of age for other than the operation of a glider or balloon.
(b) Be at least 14 years of age for the operation of a glider or balloon.
That is the age at which you can solo. Lots of airplane pilots take lessons and solo on their 16th birthday. I don’t follow the gilder world and balloon world, but I assume lots of kids solo on their 14th birthday.
To get a license you must be a bit older:
61.103 Eligibility requirements: General. To be eligible for a private pilot certificate, a person must:
(a) Be at least 17 years of age for a rating in other than a glider or balloon.
(b) Be at least 16 years of age for a rating in a glider or balloon.
One can be younger and fly with an instructor, altho the hours may not be loggable if too young. If one see out the front, one can fly. See the EAA Young Eagle program as an example. Over one million kids have been given rides, and many have taken the controls during the flight. Flying with a Private Pilot, their time is not loggable, but they enjoy the flight none-the-less.