The best age to start piano lessons is now by Prin Dumas

"The best age to start piano lessons for children is now. As a piano teacher, I'm often asked if a boy or girl is too young for piano lessons. Often, parents are told to wait until after first grade to ensure a child understands basic reading, counting and letters. I agree that it makes learning the piano easier. I disagree that anyone needs to wait to start learning about music or an instrument." summarizes Prin Dumas.

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Challenges of piano lessons before grade school
You may encounter some difficulty with motor skill coordination.
Your child's fingers may be too weak for several weeks to make a sound on the piano.
You may need to start on a weighted keyboard for some children.
Your child will need the parent to practice with them each time.
It will take longer to learn songs.

Benefits of starting piano lessons before grade school
Your child will become familiar with the instrument and not be intimidated by it.
You will find out early if they do or do not have a natural aptitude for music. Learning music is said to stimulate children's brains and help with other learning.
They will develop motor skill coordination that they did not have.
Their fingers will be stronger by the time they are old enough to play harder songs.
They will know the routine of piano lessons and piano practice before entering school.
They will get faster at learning their music. 
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Source: NJ.com