The following is from BA website:-
Making a booking for your infant or child
How old is an infant or a child?
Infants: 0 - 23 months
You must always have a ticket for your infant's journey.
Children: 2 - 11 years
Also from the website:-
Booking a child into a different cabin from yourself
There are times when you might need to travel in a different cabin to your child, and you can do this on British Airways. If your child is already 12 years old, you can make your child's booking on ba.com.
If your child is between five and 12 years of age:
- Your child can be booked into any cabin once you are booked to travel on the same flight.
- You should stay with your child until they are taken into the care of British Airways’ staff at the boarding gate.
- If you are a Gold or Silver Executive Club Member, you will be able to take your child as a guest into the lounge. However Club World, Club Europe and First class customers may only use the lounge for themselves.
Please contact British Airways to make this booking if your child is under 12 years.
And more:-
Please note:
Whilst we retain overall Duty of Care after your child has checked in for the flight, you will still be responsible for their health, well-being and behaviour, both on the ground and on board.
.......IMO, they are clearly stating that a child is aged under 12 (like all airlines) however, they go on and state that the rules for access for elite card holders & standard passengers traveling with kids.
BA clearly have a dislike for children in their lounges and as someone who has recently (this year) traveled with kids with BA in F - I can back this up with first hand experience.