I started my DS4 in Suzuki piano lessons before he turned 3. He would hear a song on the tv and then play it on the piano. He zoomed through book one, but at that point he really wanted to learn how to read music. I switched him to a traditional teacher when he was 3 1/2. He was reading books (like The Magic Tree House) and his teacher felt like if he could read that well, he could learn to read music. It's been 9 months and he has progressed very rapidly. He's gone through almost four levels of piano books and quite a few supplementary books. His teacher figured out a few months ago that he might have perfect pitch (he can tell you what note is being played while in the other room).
He recently turned 4 and you can already tell music is such a part of him. He really enjoys it and I can't imagine him not playing the piano. I agonized over the decision to put him in lessons so young. I was worried about what people would think. I was worried about pushing him at a young age. I look back now and realize I didn't need to worry about it at all. Other people can think what they want, and my son loves it so much I don't feel like I am pushing him to do anything.
It's different for everyone. You can always stop and start again later if it isn't working out. I feel lucky that I found such a wonderful teacher that is really patient with him. Good luck with whatever you decide!
