*starts fucking it* May as well get some use out of her first before you take her back if you're going to... But Yeah, if you want to see them suffer, they have to at the very least be old enough to know what's about to happen to them. Even if they don't know what sex it.
Like I said. Old enough to scream, cry, beg me not to, etc. Maybe too young to understand what sex is, but old enough to know she's taken... let's say 4-12?