Ktspktsp and I have had sex when there were people sleeping in the same room with us, several times. No walls needed. We have perfected silent sex, right down to putting on the condom.
We find it to be a great turn on, actually... and the most effective position is in a side (spooning) position, so he can slide right into me.
Let's see, the first time we did it, my old roommate was spending the night and she was sleeping on the couch on the other side of the room (in a studio). There was one moment when we made a squelching sound
but she didn't hear it (we told her about it a while later, haha).
Second time that I remember, was when we were sharing a hotel room with my older cousin... she was sleeping in the other bed, about 6 feet away from us. We waited until her breathing was regular (asleep) and then went at it.
There are other times, I'm sure, but I can't recall them right now. Maybe ktspktsp will help me out.
And of course, staying at our family's homes... at my mom's place, the bed was SO squeaky that we had to be very careful (she was sleeping in an adjoining room, with the door open between us), but we still made it happen. At his parents' place, we were in his room with his sister next door, so we had to be totally silent. Damn, that was hot.
And we have done it in several cars, usually in the passenger seat but sometimes in the backseat as well.
And yes, we are in our mid-20s.