To start playing easier songs without help, it takes usually about half a year for children, which is about 20 lessons taken in total.
This is in case when there is one lesson per week and child practises at least three hours per week at home too.
It depends how fast child develops. Usually good age to start is 7. But some children can start from 5 or 6 when they are ready.
Piano is an instrument which needs certain patience and willingness to focus for periods of time. Teaching piano to a child mainly depends if child has enough attention span to work with instrument at least 15 to 20 minutes in one go.
Age doesn’t mean that younger children can’t participate in music. Yes, they can, but learning is different.
This should be more playful and on their level. I suggest to find some musical group (for example Monkey Music) where kids sing, dance and play together. It will develop child's musical perception, hearing and rhythm understanding.
Such introduction to music gives good foundation to perceive rhythm and melody and helps to learn more sophisticated instruments in the future.
After this, starting at age 5, your child is more prepared to come and try some piano lessons.
Increase your child exposure to music: