The article noted that often the identification of the troops involved was difficult because it was too dark. If it's too dark to identify the guy, then how was the trooper supposed to be able to accurately tell the age of the hooker?
I dislike the UN as much as anybody here, but much of this is a hit job. Troops far away from home will tend to try to hook up with the local girls unless kept on a very tight leash.
Next, if your whole mission is to provide humanitarian aid obtained from other people's funds, requiring sex in exchange, or even appearing to, is hardly professional. Hopefully this isn't something that the US military would condone. Sexual bribery of underage girls is not the same as "hooking up" with the locals.