The journal is designed for elementary school aged kids. So we recommend that your child is at least 5 years old by the time you start on this.
That said, you could do it with a younger child provided you are OK with doing most of the writing and use simple language as you write so your child can get something out of it.
With kids older than elementary school age, you will have to take it on a case-by-case basis. Some middle schoolers will probably enjoy the chance to do this with you, but probably not all.
Please flip through the sample mini-journal here to see how it might work for your child specifically: https://issuu.com/afineparent/docs/mini_journal_v3