As it is a legal requirement for children over the age of two to travel in their own seat, passengers who are travelling with an infant who becomes two years old before their return flight must ensure that they have a seat reserved for the return flight. However, certain airlines ask for the date of birth for age the next birthday. Some airlines take the age they are of last birthday. So it would be best to clarify with the airline you're travelling on as the policies may differ! Hope it helps!