My son is 10 and is willfully oblivious to anything to do with sex or dating. We've had friends crashing at our place since before he can remember, so anyone spending the night seems normal to him. He treats Wendigo as something akin to an uncle since his own uncles aren't around much or interested in the same stuff he is. Loveleigh is a recent addition to our lives, but he had a blast when her kids slept over and he's sleeping over at her house tomorrow night while we all go out for St. Patrick's Day (her underage sister is babysitting for us). Runic Wolf referred to her as his girlfriend infront of him last week and if he noticed, he didn't say anything or question it.
Just remember to take your cues from your own children about what info and how much they are ready to take in.