The mitochondrial theory of aging proposes that age-related decline in mitochondrial function contributes to the aging process. Mitochondria are the energy-producing organelles within cells and are involved in various cellular processes. Over time, mitochondrial DNA accumulates mutations, leading to impaired mitochondrial function and increased production of reactive oxygen species (ROS). These ROS can cause oxidative damage and further impair mitochondrial function, creating a vicious cycle. As a result, cells and tissues become less efficient in generating energy, leading to age-related decline and increased vulnerability to diseases.