What ages are the books for?

Four bands, from three years old to teenage.

• Crayon Years, ages three to six. Coloring books.
• Question Years, ages six to nine. Activity books.
• Builder Years, ages nine to twelve. Project books.
• Founder Years, twelve and up. Business books.

The same subjects appear in all four. What changes is how much the reader is asked to do. The full breakdown is here.