How often to airlines check a lap child's age?

My husband and I will be flying with our child from Texas to Ohio. Our daughter will be a little over the age of 2 when we fly. We really don't want to buy her her own ticket. It would be a waste of money, as she can just sit on our laps, and most likely will spend most of the flight there anyway. My question is how often do airlines check the lap child age requirement?

Answer: 
Most people say airlines don't check a child's id very often simply either because it's obvious the lap child is under 2, or a child over 2 already holds a ticket. In your case, the airline will ask to see her birth certificate when your child is already over 2. Try at your own risk of not being able to get on the flight.