“Naturalized individuals have never been eligible.”
Consider this hypothetical:
In 1784, a 35 year old London-born man moves from England to Massachusetts and in 1787 he is naturalized by an act of the state legislature to be a citizen of Massachusetts.
Would he fall under the grandfather clause?
Under English law at the time, he would still be a British subject.
Under the Constitution would this disqualify him?
Indeed, as a US citizen at the time of the adoption of the Constitution, he would have been eligible.
I must cut this off somewhere. This is the last post before my final one for this discussion.
Your hypothetical was made citizen of a State under the Articles Of Confederation. The United States existed but not as currently constituted. All US citizens were disputed by England to be its subjects, leading to actions on the high seas such as impression of men in service to the US Navy, which in turn led to the War Of 1812, upon the conclusion of which the matter was settled. England had no claim upon US citizens.