My son turned two on a recent trip to Hawaii. We booked the award ticket on BA (just two tickets - myself and my husband) using AA flights, and then I called AA to add an infant to the reservation. When I explained that he would be turning 2 while on vacation, the agent said that he had to have a seat for the return flight but that they don't charge for it. It really ended up working out well, especially as they moved us to 3 bulkhead seats on the way there so he had a seat both ways for no charge. There was some confusion with the gate agents on our return flight, they didn't seem to know how to handle my son's infant with a seat reservation (he had his own reservation, not tied to ours for some reason), but after they made a few calls they worked it out.