The oldest dream that I can remember was back when I was in Kindergarten. I was sleeping on a mat on the floor, during nap time, and I had false awakening. In the dream, I sat up on my mat and looked out over the room full of other sleeping children. At the far end of the room, there was a sliding glass door (exactly like the one that was there in waking life). Then, just outside the door, I saw a gigantic figure that looked like Darth Vader come walking into the picture, outside. The glass door was only tall enough to see up to this Vader-like being's torso, but the head/helmet was actually down where the stomach was, so I could still see the 'face.'

It was creepy as hell.