One certainly doesn't need to be a child to enjoy the hobby of building and maintaining a model train layout. Many adults have this as their main hobby and spend a lot of time creating their "own little world" by way of their train layout.
If you are considering doing this for yourself or with a child, first decide what kind of layout you will have. The amount of space you have in your home, basement, etc. for the layout will most likely determine what kind of scenery you will concentrate on. The more space you have, the better it is because many model train enthusiasts enjoy eventually building things like mountain tunnels and expanding the layout.
So what kind of scenery will you have? Again depending on the layout size, you can build a country scene with trees, streams, and mountains or you can have a city scene with lots of buildings and vehicles. Some people enjoy coming up with something totally different from the norm and creating a futuristic theme, or the opposite . . . a scene from sometime in the past.
You can build a layout to look like another country or another totally different time period. This is where serious model railroading enthusiasts may be separated from those doing this activity just for fun. Of course there were no trains in ancient Egypt, but some model train people enjoy building pyramids, the sphinx, etc. for their model train to run past, as one example.
After you have decided what type of layout you would like to have, it will be time to buy your basic starter items. This includes your train and track, of course. You will also need the plywood or material you will be building the layout on.
Decide on your design and make sure that you have enough train track to make it happen. Do not attach the track to the wood or base you are using until you are positive that it is the way you want it to be.
Next you will want to decide which scenery props you will be buying or making. Mountains and tunnels are easily made with materials such as Styrofoam, plaster, plaster dyes and so on. Be sure to familiarize yourself with which accessories (i.e. trees, grass, etc.) you will be able to buy at the model train or hobby shop and which items you want that you may have to find a way to build or make yourself.
If you hadn't already read through some model railroad magazines or web sites, this is a good time to do that. You will find many excellent ideas on layouts and how to create that personal world you are striving for.
These places, whether magazine or internet, will help your project be a success without too much trial and error. For just one example, you will need to research how to make a mountain (and from more than one source!) before actually trying it. Often a mountain can collapse if everything needed is not taken into consideration and it is much better to avoid disappointments like that if at all possible.
Now it is time to decide on your grass, trees, and general scenery. Most important of all is to have fun; it's a great hobby!