I can't speak as any kind of medical professional but I've slept many nights camping outdoors and also sleeping on the floor indoors. I have always heard growing up that the cold from a bare floor can cause pain after a while (perhaps on the joints as Darrke Thoughts says), but I haven't read any studies on this.
I also agree that a person's body weight and posture has a lot to do with your comfort. I am a thin/average 29 year old and personally can be comfortable on a floor with a sleeping mat and sleeping bag, sleeping on my sides, stomach, or back.
But everyone's different so please get more opinions from other people if you are worried about waking up sore. Another trick people use with bad backs is laying on their back on a carpeted floor with a large pillow under their knees. (Looks like you're sitting on a chair, but on your back) This takes pressure off of your lower back I believe.