Itty-Bitty = perfect for those younger than 6 months old. Made to fit snugly for safety.
Infant (x-small)= for ages 6 months to 2 years old. Also, this size can fit some toddlers depending on the size of their head. If you feel like they need more room lengthwise (longer hair), then do not order this size. Order the next size up.
Toddler/Little kid= for ages 3-7 years old.
Big kid/Adult (large)= for ages 8 and up. both sizes are the same.