From a United Nations report, a chart of the prices that ISIS charges for concubines.
*Children between ages 1 and 9 includes boys.
Even if this is exaggerated neocon propaganda, the trend illuminated by the chart is likely authentic, because an ur-feminist organization like the UN would not be given to propagandizing the low sex slave worth of 40+ year old women.
There are some universal truths about the nature of the sexes exposed through revealed preferences (and pricing) that even primitive barbarians can find common ground with effete Westerners.