This book is suitable for ages 10 to 12 years. John Muir spoke, wrote, and lived the wilderness, including taking President Theodore Roosevelt on an overnight trip to the Yosemite Valley. This trip led to Roosevelt's signing into law a bill that placed Yosemite under federal control as a national park. Because of this and his founding of the Sierra Club, John Muir is credited as one of the key shapers of the modern environmental movement.