My 3 1/2 year old has been taking piano classes for almost a year. He plays little songs, read notes from the practice books while playing (C,D,E), and knows the notes by sight (short note, long note, rest, etc.). One day on the way to class, he informed me that he doesn't need to go to piano that day because he already knows how to play the piano.
To give him a sense of time-line and long-term committment, I suggested that he ask his piano teacher how long she has been playing the piano. So we arrived to class and he asked her how long she had been playing. She replied, "Almost 24 years." I asked my son how long he has been playing, and my son then replied, "20 years". He's only three and half years old!