Is a three-year-old really old enough for this?

For the concept, yes. For the vocabulary, no, and we do not use it.

A three-year-old cannot hold the term search engine optimization. A three-year-old can absolutely hold the idea that a lemonade stand behind the house sells less than one on the busy corner, because they have already lived the smaller version of that a hundred times.

What we are introducing at this age is the shape of the idea. The name for it arrives fifteen years later and lands on something already familiar.