The mother wasn’t of age at the time of the birth is what I understand the problem is with that. Besides, both of McCain’s parents were American citizens, yet his citizenship was in question because he was born on a Naval base outside the states and he had to produce his official birth certificate during the election, which he did with no qualms.
McCain’s citizenship was never in doubt. As the child of two American citizens, he acquired citizenship at birth.
The question was whether or not he was a “natural born” citizen, as required by the Constitution for someone to be President. “Natural born” is more restrictive than simple citizenship.
There’s plenty of explanation of the difference between the two here on FR. Please read so you know the difference, because it is entirely possible that it will be a serious issue in the future.