Separation anxiety is a normal phase that babies go through beginning as early as six-seven months and can last up to 24 months. Most parents see it beginning around 10 months when their babies realize that people exist even when they are not present, such as when mommy leaves to go to work or daddy leaves for the store, or sometimes even at night when your child is falling asleep or wakes up alone. In some cases, this anxiety will persist through preschool and even elementary school.