Child Fares on the Subway and Bus?

The MTA always seems to be raising its fares, which always begs the question: "When exactly do we start paying for our kids to ride the subway." I've gotten so used to shuffling them under the turnstile, it occurred to me that at some point I'm probably supposed to start paying for kids to ride the subway.

Well, I looked it up and children are supposed to pay full fare when they are over 44 inches. By my calculations that means that you should start paying for your child when they are about five-years-old. Um, right, perfect timing... for my second child.