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