I don't remember the exact reason of why the youngest and presumable the most healthy were the biggest victims, but I think it had something to do with their more active immune systems actually working against them. I think their immune systems essentially went into overload sending them into shock or something...
Much like you might think that younger men, in their 20s, would be more likely to survive a ship wreck. I believe studies done by the Royal Navy after WWII that proved otherwise, that middle aged sailors in their 40s were far more likely to survive due to a lower metabolism and a life experience that showed that adversity wasn't necessarily the end...