What age is this for?

The child-level explanations are written for roughly three to nine, and the grown-up sections underneath them are written for whoever is reading aloud.

There is no order to follow and no age at which a subject becomes required. A three-year-old will take the lemonade stand and ignore the rest. A nine-year-old will argue with the search engine explanation, which is a better outcome than agreeing with it.

If something lands flat today it costs nothing. Try it again in a year when a cartoon happens to mention the same idea.