1. Infancy spans from birth up to 15 months old, where babies primarily communicate through crying and develop senses like sight, smell, and taste. 2. From 3-4 months, babies can lift their head, smile, and reach out to people. Between 5-6 months, they can sit up, hold objects, and show attachment to parents. 3. By 9-10 months, babies can crawl, identify people, and start to stand alone or take a few steps between 12-13 months with assistance.