Many good insights above. Music is a big part of our household. (SO takes the position that everyone will learn the piano in some form--no option!) I would not be surprised if reading is a key portion of the change for your DC.
Our DC do love music, including the piano, but reading music is not a popular activity in our house, and has been a challenge to develop, since it only take a repetition or two for kids to memorize, or they learn pieces by ear. We have one who has progressed to being able to read, more or less (one new piece every day from Bela Bartok's Mikrokosmos, over a period of about a year, in case you're wondering--no repeats (so no memorizing), and not-quite-diatonic music (so less surprisingly-accurate-guessing)), though rather slowly, and another who is essentially illiterate (musically!), but picks songs up very easily by ear. I have had to change the way I teach the respective children, because if I forced the non-reader to learn predominantly according to my classical training, a lot of the joy of piano would be lost. Left to DC's own devices, there was plenty of time on the piano, just not practicing what I'd assigned. At this point, we've switched to learning to play by chords, learning how to learn a keyboard part from a recording, etc. A summer session of lessons from a rock keyboardist was well received.
Oh, and none of our musically-inclined children consented to begin piano lessons from either parent (both experienced pianists) until age six. Though they were always on the piano constantly, experimenting.
