What age group is this book intended for?  That really depends on where the child is in their comprehension and literacy.

A five year old who has been read to previously and is developing their auditory literacy very much enjoys and understands most of it; explain the difficult words and sentences.

A toddler benefits from being read to from a book like this, even though they don’t understand it. They begin to hear new words, improve phonics of the ear, and grow their literacy and their bond with the loving person upon whose lap they sit.

A teenager or even an adult who is struggling with reading comprehension, who finds The Desire of Ages difficult, could start with this book as ‘entry level’ Ellen White from which they can grow into the full-length books by Ellen G. White.

So, short answer: potentially all ages! Depends on where the reader or listener is in their personal literacy.