In France it is not legally mandatory for adults or children, though strongly encouraged. You'll find most people on the slopes wearing a helmet.
Why You Should Wear One
Safety: Even if you are an excellent skier, accidents are frequently caused by collisions with other skiers or unexpected obstacles.
Warmth: Modern ski helmets are far warmer, better ventilated, and lighter than traditional wool hats.
Insurance: Some travel insurance policies require you to wear a helmet for your coverage to be valid