Music is a Universal language. Just like any language it is easier to learn at an early age.
I believe in starting music education as early as three years old, depending of course on each child's level of developement. Beginning at such a young age, encourages a child creativity and self confidence and has been shown to develop better cognative skills.
I believe that if a child is not having fun they will not be as receptive to learning, but music requires discipline and consistent practice is very important in order to master each level.
Every child is different and will progress at their own pace, but with attention and practice every student can master the piano.