My daughter, aged 12, is keeping a Commonplace Book. I have absolutely no say in what she chooses to put in there and I never “check” it. Once a week, we have a family meeting and she shares things from her book. We talked about this together several times over a few months before she started doing it. I also showed her a couple of websites for inspiration. When we were reading Great Expectations together, I would share with her from time to time some of the things I was writing down. Pretty soon, I noticed she was writing in her book regularly.
I have not done any extensive reading on the Book of Mottoes so I cannot speak authoritatively about what it should or shouldn’t be, but I believe it is comparable to a Commonplace Book. I did not set any expectations or rules for her book. Rather, I tried to inspire her to think of it as something that could be kept for her whole life and something that was wholly her own. She does not categorize it, but she does write down the title of the book and the chapter from whence the passage came.
My own books are categorized. I actually keep separate books for Science, Education, History, Theology, and Literature. I find this to be easier for me because without this system I would be paralyzed by not knowing where to write something and I would end up just not doing it.
By the way, my son, who is 11, is definitely not ready for this. He is just doing written narrations for now and no copy work. His handwriting is fine and he hates copy work, so I think written narrations are best for him. He also does prepared dictation with Spelling Wisdom. I think I will know when he is ready, just as I knew when my daughter was ready. I could see a maturity in her, a sense of wanting to spread her wings and take some responsibility for her own mind-nurturing. I’m sorry I don’t havesomething more practical to tell you about determining their readiness.