The Quran may promise virgins but it does NOT state either the age or sex of those virgins
An 90 year old MALE shepard who has lived all his life in the mountains might well be a virgin
Actually I do not remember the Quran even stating whether or not the virgins will be human
Also you should beware of those you kill - in Islam when you kill someone you take on the guilt of their crimes and have to answer for their crimes as well as your own - definitely something you should think carefully about before taking a life
Perhaps you as murderers should ask your potential victims the question before you slaughter them:
"Are their any crimes you would have needed to atone for upon your death?"
As those crimes will now become yours to answer for and your victims will be totally guilt free upon their murder