We've been reading through Paddle to the Sea this year with my 6yo and 8yo... the "plan" was to read a chapter a week, to give sufficient time to cover some of the extra information during that week. The boys love it so much, though, that sometimes we read more than we planned :D . At this point, I am content to just touch upon some of the extras - I figure we like the books enough that we will cover them again down the road. The main extra thing we are doing now is mapwork, plotting Paddle's course as he travels to the sea. We are keeping a list of all the wildlife he encounters along the way, and I plan to have the boys each pick one or two for a more in depth study.
As to your original question about the most beneficial age, I think ALL ages will benefit. If it's basically a read aloud right now, that's great! :) My 3 yo will sit and listen while I read to the older ones, and he is enjoying it. The 8 yo is probably mulling over more than the 6yo, but it is of value for all of them. Use it now, then use it again.
I do not have the Beautiful Feet guides for the Holling books, but they look splendid. Maybe in the future...