My dd1 (12) is very good about going to her room at nine-thirty, and is welcome to read/ watch a dvd (she doesn't do that often though) or generally attend laboriously to her grooming as she is starting to take an interest in that now. I do say to her though that that's it, no coming downstairs because it's time for me and my DP to spend some time on our own together. I have used pretty much those words too, as I think she's understanding enough to be respectful of that.
It's trickier with my dss (13) when he sleeps over though, as he has been spoilt used to staying up longer, and it took him a while to be comfortable leaving me and his dad alone together. It got there over time though, and to get some adult time in the evening now, we find it is much easier if we do something fun together, but first make clear the deal is bed by ten (sometimes get away with half nine, but it's rarer now.) So we'll all play a good board game or quiz, which, although it sounds terribly Victorian and they are savvy game-stationed-up kids, they actually love, maybe because it's a novelty to them, and they are getting our full attention. We are joking and being daft altogether, and it's lovely to see their sense of humour and the kind of young people they are turning into.
By ten or whenever we'd agreed, they are relaxed and good-humoured and happy enough to slope off upstairs. In Dss' case, his dad goes up with him and puts a dvd on for him(it's just the habit he has of how he goes to sleep), spends a few minutes pottering round with him finding stuff for the morning etc and dss drops off in his bed watching that.
DP gets back downstairs a few minutes later and I have a bottle of wine open - ta-daa! Then we spend at least two hours together on our own with a movie or just chatting. If we are positive we can't hear the floorboards creaking from upstairs , we might get quite cosy ...
All in all, I think the principles for older children are the same as for younger, just with adaptations. You need a bit of a routine, you forewarn them when it's time to go up, and you make them feel welcome to you and that you enjoy their company before you boot them upstairs, erm, I mean bid them goodnight! I don't see anything wrong with them having a mooch about on their own in their rooms til they are tired, but I have no problem letting them now that that time downstairs is for me and DP to be by ourselves too.