People often think that children should start chess early, and in some cases (for example the Polgar sisters, Samuel Reshevsky and Arturo Pomar) they’ve progressed quickly as a result. On the other hand it does seem as if the majority of kids find it too difficult before the ages of 9 or 10, so they are put off playing never to return.
I think this is an important thing to realize, we’re all different and develop in different ways and at different speeds. Starting chess later also seems no barrier to players reaching great heights as many Grandmasters developed as teenagers rather than primary school children.
Here anyway is an example of starting them young. The little girl identifying the pieces is 2.5 years old, somehow it doesn’t surprise me that the mother has a Russian accent: