From my experience with hockey, it can be a fairly dangerous sport. This is no reason to stop a player of any age to try and learn and play the sport, however, it would be best for players of all ages to wear full face protection. Being injured in a game can have life long consequences so it's best to prevent as much as possible.
Because it protects their face from fists and speeding pucks. If they don't want to have plastic surgery to fix their faces after being hit by a puck flying at over 80kph, or being elbowed full in the face during a fight, they should wear full face protection. This protection should be made mandatory in any and every hockey league!!!!!!!!!!
They have the choice to wear full face if they want so tis is pretty much an irrelevant topic because players are allowed but they choose not to because it impairs vision and some players do wear full face protection. Many players who have had a face injury such as a broken jaw.
I am a high school hockey player, and I am an avid hockey fan. I believe that the current rules and regulations throughout the hockey world are good enough. The new rule grandfathering in visors in the NHL is a good rule, but those small visors that only cover the upper face are good enough. Pros have made it clear that it impedes their ability to see the action on the ice as clearly, which can be a safety hazard in such a fast game. Introducing visors will also lower the integrity of the professional game.