Nearly all children are walking by 18 months. Baby can be used to refer to any child from birth to age 4 years old, thus encompassing newborns, infants, and toddlers. They learn to smile, roll over, sit up, wave, clap, pick objects up, crawl, babble, and some may even start saying a few words. As babies move into their second year of life, they become more mobile and more independent, exploring everything they can access. However, if you are worried about your baby's development, check with your pediatrician. Toddlers may be considered children that range from 1 year to 4 years of age, though others may have different definitions of these terms.