That's a very good question, because I don't think it's ever been a real issue. Not among "serious" candidates (read: not some rando third party with zero chance).
Those factors are more mechanical. Being of age isn't a judgement issue unless someone says there's a mistake on their birth certificate or something. Natural born citizen is usually mechanical. Most candidates have been clearly born in the United States, and there's no debate about it. There was a little question around McCain, who was born in Panama, but there was clearly an act of Congress that made kids born there to American service members into citizens. If we take all the claims of the Obama Birthers at face value (which are nonsense), then he'd still be a citizen due to his mother being a citizen.
However, I do think the Birthers uncovered a problem while they fumbled around like idiots. They tried to get Obama off the ballot using the courts, and repeatedly had them dismissed due to lack of standing. If there actually was a valid reason to challenge someone under those requirements, it's not obvious who can enforce it.