When is the acceptable age for a child to learn to play the piano?

-My friend has a 3-year-old daughter.The daughter is interested in music.So I am wondering if my friend can start piano lessons for her daughter now or whether she needs to wait.I would appreciate it if you can answer me based on your personal experience.I never learned how and I feel it would have been a lot easier to start at a young age because you can learn ahead of time if it's what you really want to do. If the daughter wants to, that's great, it's great to have a passion at such a young age!
Singing along with Barney does not translate into "gifted pianist". Let the poor kid be a kid and enjoy life for a while.
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