I started playing piano when I was 4. Was it the best age? I dunno, that's just when my mom signed me up for it. But all this about waiting until a kid shows interest, I never quite understood. How could a young child show interest in something until they've been exposed to it? And really, as parents, aren't you choosing what you expose your kids to in some regard? A little OT, but something I never quite understood.
Anyway, I can tell you that with my experience, playing at such a young age, I can't actually remember a time when I didn't play piano. Reading music is as easy to me as reading a book, and I can't actually remember a time when it wasn't that easy. And because it was something I just grew up with, I didn't find it odd having to practice, it's just what we did. Granted, as I got older and realized that not all my friends had to play piano for an hour a day, there were fights with my mom over it, and she did force me to still practice. At the time, I hated her for it, but now I'm really grateful that she made me stick with it. It's a great form of expression, and I personally love music, so if I hear a song I like, it's great just to be able to sit down and play that same song. Plus, it makes the holidays so much more fun.
Just my 2 cents, for what it's worth. But I would say the earlier, the better.