Your little mountain rider will be ready for the trails wearing the Giro Youth Tremor Bike Helmet. It has 18 vents so they stay cool and an in-mold construction for a lightweight, durable design. FEATURES: Full hardbody shell Ventilation: 18 vents In-mold construction fuses the polycarbonate outer shell with the EPS foam liner Ponytail compatibility Complies with U.S. CPSC Safety Standard for Bicycle Helmets for persons age 5 and older SIZING: UY: 19.6"-22.4"/50-57cm