You've gotten a lot of good advice. I teach music lessons (piano and guitar), so I thought I'd chime in. I'd say that those who said that it depends on the child, teacher and method (Suzuki or otherwise), are totally correct. I've seen some students start at 5-6 and do great, and other who would have benefited from waiting until they were 8-10. And one of my best students didn't start until he was about 11-12, and was playing Chopin Nocturnes in 3 years. (fyi - I started learning piano at age 6.)
Age, reading ability (if not doing Suzuki), motivation, and concentration are all factors. So is the general musicality within the household. And parental participation/encouragement is critical at younger ages. The problem I've found with starting kids too early is that they don't have the discipline to practice well (and end up just going through their songs once a day or something, and not really progressing in a way that makes music study satisfying to them). Another problem is that the coordination and multitasking involved in learning piano (2 hands, 1 foot, notes, rhythm, dynamics, etc.) is often overwhelming for little ones.
Something that I suggest to friends with kids (and am doing myself with my 2 year old boy) is encouraging kids to just play with instruments, and to sing. Singing is wonderful because it can be done anywhere, requires no reading or purchase/rental of instruments, and is a great foundation for learning rhythm and melody.
Best of luck!