What is the absolute youngest age limit to fly a plane?

In the United States you have to be 17 to receive your private pilot certificate. You have to be 16 to solo as a student pilot. You can, however, fly a plane at any age as long as you have a certified flight instructor in the plane with you.

I have known 9 and 10 year-olds who were taking lessons. However, they had to wait until age 16 to solo and 17 to take private check ride.