It’s written to be used with a wide variety of ages. Nature study is applicable to everyone, but it will look a little different depending on the age of your children. I know people who use it with their three-year-olds and I know parents who use it for themselves.
There are journal pages included, which you can skip completely if your kids are really young. They can also do the drawing and they can narrate to you while you write it down. This is actually a great pre-writing activity. Older kids can also keep their own nature journals if they wish. Having said that, while I feel like recording your time in nature can be a rich experience, I don’t feel like it’s a necessary part of the program.
Here’s an example of an activity about observing insects in a patch of wildflowers and how it can span a wide variety of ages. At the simplest level, this activity would be to see how many insects you can find on a patch of wildflowers. You can expand from there with links included in the study and journaling about it.