If Barry had been born in Kenya to Stanley the mother and BHO Senior the father, he was not a US Citizen for the reason you set out--residence of the citizen mother in the US for five years after age 14 was a condition of citizenship to the child and Stanley would have been, under the hypothetical, only 18 at the time of birth in August of 61 and therefore could not possibly have met the requirement of the condition.
Subsequent modifications to the statute which would have modified the result contained a specific exception clause in the effective date provision of the Public Law that made such modifications effective only with respect to births after the effective date which in all cases was after the purported birth date.