Combi Fare for Kids

Does anyone know how old kids have to be to start paying combi fares?

When they take up space. So, no charge for in the arms or on the lap. Standing between you legs? Yes.
They are very inexpensive anyway.

Signs in combis say above age 3 must pay the fare. (But, it appears that enforcement depends on the driver.)

When they take up space. So, no charge for in the arms or on the lap. Standing between you legs? Yes.
They are very inexpensive anyway.

When I read that they are very inexpensive, I couldn’t help but think of Joliet Jake Blues.

https://youtu.be/rKHK5taD0FM?si=2obybgnPfWMYlBEd