We brought our 7 month on an Allegiant flight last Nov and the check in agent asked for his birth certificate. At the time their rules stated an older age (I think it was 14+ months) that would need a birth certificate (I just checked and now they require it for all lap children) He is a big guy, but how many 2 year olds do you know with 6 teeth? They let us on. The next flight was at 13 months with Spirit, we brought his birth certificate, and did have to show it. This flight at 17 months we will fly Southwest, and purchased him a seat. At 13 months it was a tight fit with him on our laps. I don't think we need a birth certificate this time, but I will probably bring it just in case.
Donna