Well, ... that's a good question
I didn't necessarily have an age target in mind. However the starter phonics book
was designed for my younger elementary school students. And now I use it with k-2 students to start. After that we just progress as we can through the series.
I had some students who begin with me around 3 years of age. I felt some groups were ready to start some phonics but maybe not ready to start writing. So, for them I made the ABC book
which has an introduction to the alphabet and phonics without letter writing.
I assume most EFL book 4
users will be at least 3rd grade or, more likely, upper elementary school students. If the students don't have a good vocabulary foundation, a lot of the reading parts will be difficult. Even though the students might be able to read, they won't understand.