If you’re booking a hotel room, usually a child is considered to be 15 years or under, unless otherwise stated. Those 16 and over class as adults.
For lounge bookings, a child is considered to be 11 years or under, so those 12 years and over would need to be booked as an adult. This can vary from lounge to lounge, so make sure you check by clicking on the blue 'Info' tab for each one.