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.