I started at 5 and I was by no means a prodigy. It really depends on the individual child. Some are just more able to sit still and concentrate than others of the same age. So I would watch the child carefully and if she demonstrates that she can sit still and concentrate on one task for 20 minutes or longer, and/or if she shows an affinity to music (singing, dancing spontaneously or to songs on TV, CDs, etc.), then I would definitely start her off fairly early.
It's been found that there is a narrow window of opportunity for children under 7 learning a second language, arithmetic, and music more readily than if they were to start later. The Suzuki method allows very young children (as young as 3!) to start learning music in a group setting. Personally, I feel 3 is too young, but some parents can be quite aggressive in pushing their children into early music lessons. For everyone's sake, take your cue from the child.