Acharya Prashant explores the definitions of childhood, youth, and old age from a mental rather than a biological perspective. He defines a mental child as someone who is dependent and lacks the capacity for independent thought or decision-making. This dependency is not limited to physical needs but extends to a mental reliance on authority, guardians, or societal conventions. He argues that many individuals who are physically adults remain mentally children because they cannot think for themselves and instead live a life of obedience and submission to external influences.