There are minimum ages for the various licenses and permits you might be training for (16 years old is the minimum and applies to a Recreational Pilot Permit), however there is technically no minimum age to begin training.  There is a requirement to be at least 14 years old to hold the Student Pilot Permit that is required before a student can fly an aircraft solo for their pilot training, but otherwise the student should be old enough to handle both the physical and academic demands of the training.