Why Grow Old by Orison Swett Marden is a motivational book that explores the power of mindset, habits, and lifestyle choices in maintaining youthfulness and vitality. Marden, a well-known self-help author, argues that aging is largely a mental and attitudinal process rather than just a physical one. He emphasizes the role of positive thinking, purposeful living, proper diet, exercise, and avoiding worry as key factors in preserving one's health and vigor. Through inspirational anecdotes and practical advice, Marden encourages readers to embrace a youthful spirit, defy unnecessary aging, and live with enthusiasm and optimism.