All I can say is that yes you need to purchase a seat if your child is 2 yo. Now that being said, I have traveled with both of my sons the youngest 3 times under 2 and with the oldest only 2 times on international, and we were NEVER asked for birth cert nor proof of age, not even from TSA agents. Now that my youngest is turing 2 in May I will be paying for a seat for him in June, however, I am sure he will not use it only my lap sound asleep.