Ready or not, here it comes - the time for kids to leave the nest and board a bus for school. But how do you know if your child is ready? Age is the first consideration to take into account. In most public schools and many private schools, children must be at least 5 years old before entering kindergarten. The cutoff varies by state and region, but it can range from July to January, so it's best to check with your school system well before fall rolls around. Many systems focus heavily on school readiness factors such as physical, social and cognitive development more so than age, however, and these factors are emphasized if your child will attend a private school. Children born prematurely sometimes need extra time to develop, but the child's size should not necessarily be a factor in sending him to kindergarten.