Youth: 51cm - 53 cm
Lg: 57 cm- 59 cm
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.
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.