Your question relates to the developmental psychology of children more than their capacity to decode text. As such it has very little to do with chronological age because there is so much variation across individuals.
Even though publishers (or booksellers) may want to use labels such as 5-7 year-olds, these are essentially meaningless. The issue is not simply variation across the population because the accessibility for an individual of books written in similar styles will also vary with content.
A reader fascinated by dinosaurs will keep track of four plot lines in three time periods involving 10 Latin names with apparent ease, but then be completely stumped by what (to an editor) seems a simpler book about flowers or fish. The difference lies in background knowledge and intrinsic motivation.
The classic works most often cited are those of Jean Piaget (although he has tended to fall out of fashion). If you are interested in how children handle the progression in complexity of situations, I recommend the SOLO taxonomy of John Biggs as a starting point. Although it was developed for categorising assessment and curriculum tasks, the underlying understanding of number of elements and their inter-relationships applies well to plot construction.
One possible use is in suggesting mechanisms for reducing the demand of a text that you have written but find too complex for your target audience. SOLO gives you a basis for identifying the features that are creating the barriers.