Assignment of Staff. One or more staff members shall be assigned to each group of children as needed to meet the requirements for group size and staffing set forth at §§C—E of this regulation. Age of a Child. For purposes of determining group size and staff required under this chapter, the age of a child is determined solely according to the date of the child's most recent birthday. Same-Age Groups in Nursery Schools. In groups of children of the same age in a nursery school, the following requirements apply: Same-Age Groups in Before-School and After-School Child Care Programs. In groups of children of the same age in a before-school or after-school child care program, the following requirements apply: Mixed Age Groups. A mixed age group with preschool children may not exceed 20 children. In a mixed age group with preschool children 3 years old or older, the staff to child ratio is 1 to 10. A maximum of six 2-year-olds may be in a mixed age group of 13 to 20 children. In mixed-age groups of varying sizes that include children who are 2 years old, the following minimum staffing levels apply: When children who are 2 years old, 3 years old, 4 years old, and 5 years old are grouped together, the average age of all the children in the group as of September 1 of that school year is used to determine the group's maximum size and staff/child ratio.