Getting full mobility in your wrist extensor is important because it allows your wrists to move properly in the backswing so you do not have an open face at the top of your backswing
Teach kids that with irons you must hit down on the ball to make it go up because the ball gains backspin by rolling up the face
There is no set age to start a child in golf; if they have an attention span and an ability to comprehend then they are ready to learn