Final answer:
Infants typically start smiling at around 6 weeks after birth and reach other developmental milestones like babbling and crawling by 6 months. By 12 months, they may say their first words and can stand or walk with support.
Step-by-step explanation:
Infants typically start smiling at around 6 weeks after birth.
By 6 months, they are babbling and have learned to sit and crawl. By 12 months, infants may say their first words and can stand with help or even walk.