You can go the Regulations for state to get the definitions of age groups.
I have attached a copy of Massachusetts Regulations.
Don't know if this helps you.
Field Supervisor & Billing Specialist
National CCFP Forum Leadership Council Member
360 Merrimack St. , Suite 210
Lawrence, MA 01843
781-275-2720 Ext. 124
Infant: Birth to 15 months
Toddler/Two: 12 to 36 months (1 or 2 years old)
Preschool: 30 months – 5 years (not in Kindergarten)
Kindergarten: Kindergarten refers to children enrolled in a public or private kindergarten program that directly precedes Grade 1.These age ranges purposefully overlap. Children between 12 and 15 months may be categorized as either Infants or Toddler/Twos. Children between 30 and 36 months may be categorized as either Toddler/Twos or Preschoolers. If a class includes children whose ages range beyond the overlapping portion of two age categories, then the class is a mixed age class.Many school systems now enroll children aged 3 or 4 years old in classes labeled "kindergarten" that encompass preschool curriculum, not traditional kindergarten curriculum directly preceding assignment to Grade 1. Those classes all fall within the preschool age category.