Final answer:
Describing oneself in terms of age, gender, and physical size is known as Self-concept. This concept starts to develop in early childhood and is instrumental for healthy psychological development. It continues to evolve throughout adolescence as teens form their identity.
Step-by-step explanation:
Describing oneself in terms of age, gender, and physical size is the milestone known as Self-concept. Self-concept is an understanding of who one is, starting from when a toddler recognizes their own reflection in the mirror as themselves, progressing through childhood as they compare themselves to their peers and recognize their own personality traits, and being re-evaluated in adolescence as teens form an identity. The foundation for self-concept is laid out in early childhood and is crucial for healthy psychological development, impacting confidence, school performance, independence, and the willingness to try new activities.