[Non-Sequitur]

As I understand the US Code, if a person is adopted, as a child, by a foreigner in a foreign country, that person can affirm their US citizenship upon reaching 18 years of age.

No, other way around. The person can give up his citizenship when he or she reaches 18, not affirm it. If they were born in the U.S. then their citizenship is a birthright that cannot be taken away from them, cannot be given up on their behalf while they are a minor, and cannot be relinquished by accident - the action must be deliberate. And there is no window for that. An adult U.S. citizen can give up their citizenship at any time.

[WOSG]

The person can give up his citizenship when he or she reaches 18, not affirm it. If they were born in the U.S. then their citizenship is a birthright that cannot be taken away from them, cannot be given up on their behalf while they are a minor, and cannot be relinquished by accident - the action must be deliberate. And there is no window for that. An adult U.S. citizen can give up their citizenship at any time.

Wow. A correct statement! Like an oasis in a desert!