Infant: 19in - 20in (49-51cm)
Youth: 20in - 21in (51-53cm)
Typically, infant fits 6 months to 2 years. Youth usually fits 2-8 years old. Although, my 11 year old girls still fit into youth.
Emma is 2 years old and fits into a infant (about half way through the snaps) I would say they run big.
Keep in mind all noggins are different and the only way to know if our hats will fit your child is to measure. Measuring is easily done but using a string, wrap it around the largest part of the forehead. Then measure how long the string is.