I don't know if it depends who you're flying with or not, but we were told by Virgin to put the child's age when departing.
With cosmos I had to book a week earlier than I want as my son would have had to pay for his own seat had he turned two while we were there. As it is now he's free for everything
i was looking at the link for the august flights u posted thru kayak my sons birthday is the 10th aug so if i go for those cheap flights he will turn 7 while away