Many of us have heard of teenagers who soloed an airplane on their 16th birthday.
These teenagers probably started flying years before they were eligible for a student-pilot certificate.
14 for soloing in gliders.
License for balloons and gliders is 16.
Earliest solo flight in powered flight (gyros, helicopters and fixed wing powered) is 16.
Earliest license for fixed wing powered flight is 17.
You could google it or I would be happy to post the exact FAA regulation as needed.