Age requirement for dual
You can receive dual instruction at any age. However, pursuant to
14 CFR 61.83, you have to be at least 16 years old to obtain a student certificate:
Sec. 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.
Of course, you need a student certificate to solo an aircraft.
As a practical matter, a student pilot in the airplane with his/her instructor is a passenger. As was the girl from several years ago. The instructor is always PIC on a dual flight with a non-rated student. By that, we mean a person who holds less than a recreational pilot certificate and/or is not rated in that category and/or class of aircraft.