Hi
Kids can learn from 5, if they are keen and interested, but I think formal lessons too early for the average child can put them off. Music should be FUN at that age, not hard work!
But there are other things to consider: can she sing in tune? Can she pick out a recognisable tune? Can she make up little tunes of her own, that sound reasonable? Starting on Middle C can she sing to the piano, going up one white note at a time? (Don't use black notes at this stage.)
As a Teaching Assistant in primary schools I have been involved in helping kids enjoy music for over twenty years: ran a keyboard club, recorder groups, coached kids on percussion to play for the Christmas production. But note I say ENJOY, rather than learn!
And listening is also important. Not just having the radio on all day, but selective listening whether it is ZingZillas, Lazy Town, or the Proms, (currently on radio, and some TV.)
Do adults in the house play at all?
The John Thompson children's piano books are easy enough for any parent to help their child start to learn the basics of piano; I don't know the current editions of these, but years ago 'Teaching Little Fingers To Play' was one of his introductory books. They have pictures, cartoon characters and are very 'user friendly'!
If you want to reply to some of my queries, I'll see if I can help a bit more.
Cheers