If our lap infant turns 2 between the departure date and return date of a roundtrip flight, do we need to buy a full ticket for her? This would be a domestic US ticket. It doesn't explicitly answer this at co.com. I could imagine the answer being 1. Yes, you need to buy a roundtrip or 2. You need to buy a one-way for the return or 3. No, it depends only on age of infant at departure. Someone I know claims 3 was true for her but of course I'd like to confirm.