Final answer:
The UN has a zero-tolerance policy on sexual exploitation and abuse, not typically considering the mistaken belief about age a valid defense, particularly when the person involved is under 18. UN personnel are expected to maintain higher standards of conduct, and claiming ignorance of a person's age does not usually absolve them of responsibility.
Step-by-step explanation:
Regarding the question whether claiming that the girl lied about her age is a valid defense in a UN investigation for sexual exploitation and abuse, it is important to note that the United Nations has a strict zero-tolerance policy regarding sexual exploitation and abuse. This policy applies to all UN personnel, including contractors like Khalid. Thus, irrespective of the girl's alleged misrepresentation of her age, the UN generally does not consider claims of a mistaken belief about age as an exoneration for acts of sexual exploitation and abuse committed by its staff or associated personnel.
Furthermore, the UN’s focus is on protecting vulnerable populations, particularly in conflict or post-conflict areas where the power imbalances are stark. Given that women and girls often face the brunt of violence, including sexual violence, it is the responsibility of UN personnel to abide by higher standards of conduct to prevent such abuses.
Therefore, the defense that Khalid believed the girl to be of legal age is unlikely to be deemed valid or sufficient in the eyes of the UN, especially when it involves a person under 18, which is considered a child in the context of sexual exploitation and abuse allegations.