Building a model one time, my workbench (computer desk) was packed with junk I was working on. There was a knock on the front door, and I was holding the model in my hand. So I got up, set the model down, went to answer the door. A new kit that I had been waiting for had just arrived! YAY!!! Came back in to my desk, and sat down... CRUNCH!!! I had set the model on the chair because there was no room on the desk and had completely forgotten that I put it there...
As for age, I don't believe it has anything to do with DUH moments... All my DUH moments are courtesy of all the times I banged my head on a solid surface, right after a DUH moment!!!