A child does not need to read music, sit perfectly still, or already know an instrument before starting lessons. Readiness usually shows up in smaller ways: they are curious about music, they can follow a short direction, and they are comfortable trying something new with a kind adult.
For many families, ages 7 to 15 are a natural window because students can usually understand patterns, practice with a little independence, and enjoy seeing progress. Younger students can still do beautifully when lessons are playful, short, and confidence-building.
The biggest sign is not talent. It is willingness. If your child likes music, enjoys encouragement, and can handle a simple routine, a free trial lesson is often the best way to see what kind of teacher and lesson pace fits.