To get more to the point of what the founder’s intended in the Constitution, I would do a simple test. At the age of 18, could the person in question have chosen to become a citizen of a country other than the United States? As a matter of law that person could have divided loyalties.
I may be wrong, but I believe that Rubio could not have become a citizen of Cuba at the age of majority.
But all that aside, the Dems have trumped the citizenship issue for any candidate until 0bama is declared an unlawful President. Because he clearly had the right to citizenship in another country by right of birth. Hell, me may still have the right to do so.
Answer: yes, he could have become a citizen of Cuba at age 18.
If England decided that any U.S. citizen at age 18 was eligible for English citizenship - would that render every American ineligible for the Presidency?
If China made a law that anyone with over 50% Chinese ancestry was automatically a citizen of China - would that render ineligible all Chinese Americans?
Not relevant unless the person chooses not to be a US citizen.
There are countries that have weird laws granting citizenship to natural born Americans, depending on such conditions as having a grandparent of their nationality. However, these foreign laws are of no consequence as to the eligibility of natural born Americans. Obama is an example: because his father was Kenyan, he could have claimed Kenyan citizenship. But he didn't.