In the United States, most children in first grade are between 6 and 7 years old. However, there are some exceptions to this rule. In Idaho, for example, children must turn 5 on or before September 1st to be considered "of school age". In some states, children can start first grade at the age of 5 if they meet certain requirements. In Mechanicsburg Area School District, children registering for first grade must turn 6 years old on or before September 1st. It is important to note that age requirements for first grade may vary by state and school district, so it is best to check with the local school district for specific requirements.