It's not quite a universal rule, in fact it's not a rule at all, however many airlines will permit children under the age of 2 to travel with a guardian for no cost or a very low cost.
For example, Air Canada has applied minimal fees to our past clients travelling with infant children, provided the child is seated on the lap of the seated passenger.
The precise rate varies by airport or destination, but typically a small amount which would account for taxes or any insurance obligations by the carrier. I've yet to see a fee over $32.