This may not have been true in 1966, but it was true when I was in the Army.
The minimum age for enlistment was 17 years of age, and to enlist at that age required the consent of a parent or legal guardian.
If an individual enlisted prior to the age of 17 years, or was 17 but had forged the consent forms, it was considered a fraudulent enlistment.
HOWEVER, if such a fraudulent enlistment went undiscovered by the Army until the person had attained the age of 18, it automatically converted to a legitimate enlistment and became fully binding upon the person who had enlisted.
Example:
Imagine the case of a person who enlisted at age 16 years and 9 months by claiming to be 17 years and 9 months. Even if they had a parental consent form, this would have been a fraudulent enlistment, because they would have misrepresented their age.
At any time prior to their actual 18th birthday, they could escape the remainder of their enlistment by admitting the fraud. However, if they waited until the day after their 18th birthday, they would be obliged to complete the term of their enlistment.