3.5 years+
We all know how difficult is to 'teach' the 3D shapes to young children. In time, I discovered a way to do it, actually to 'watch' it as the children will create their own 3D shapes so that they understand the difference between them and the 2D ones.
Practically, we use the same technique as for the balancing structures, and for this, I recommend you do the balancing structures some time before you have the 3D topic, so that they master the technique and be able to build their 3D shapes on the knowledge and abilities gain from building balancing structures.
It's better to choose some large beans, I chose whole broad beans, put them in a bowl, pour water to cover the beans and leave them overnight to stay and soak. The next morning, you can start building your 3D Geometric shapes, using toothpicks and the soaked, soft beans.
The way I did it was to build the base first (triangular or square base) and then to stick in some more toothpicks to build it up. Depending on the age of the children, you can add some more features on your 3D shapes. Obviously, you can talk how different are they from the 2D shapes! im sure they will notice the differences!
Have fun!