How old is a kiddie bike for kids to start learning?

It depends on the child's height.

Children between the ages of 2 and 7 can ride a balanced bike, but height (especially long legs) is very important. After all, small feet can step on the floor is the basis of smooth sliding ~

If the height is not high enough for the time being, you can also use the body reducer to extend the length of the balanced car. So the car is more cost-effective!