Although I've been "training" since I was 4, I never had a permanent layout till later. From 4-6 I had several Marx sets, both clockwork and electric. They all ran on track circles. At 6 I received my first real model, an AF "dual" train set. Had 2 locos, about 12 cars, 2 cabooses, and 4 turnouts. I was basically "designing" layouts even then. Everytime it got set up there was always one passing siding, and a 2 track yard of some kind. Could never have anything that stayed up all the time cause we never had the room.
My first layout in HO, when I was 8, was a very simple affair. A simple oval of track with a Tru-scale #4 switch built from a kit. The entire size was just about 3.5"X5". It had to slide under my bed when not in use. I had no ground cover or structures on it, but I did some experimentation with some homemade trees made from 30ga. steel wire twisted together, painted brown and then covered with green lichen turfs. In my imagination, those trees turned the layout into one that traveled thru the dense forests of the south, as the fast freight worked hard to stay on schedule to get to its destination. My only loco at the time, a Rivarossi 0-4-0 dockside had no trouble pulling all 6 cars I owned then. Since then I have had 7 more layouts, each a lot better than the one that preceded it.