I think they're confusing the exit row with the middle bulkhead where the bassinets are.
If you go to the seating FAQs at
https://www.britishairways.com/trave...ng/sub/Seating and expand the exit row seating question you will see
Originally Posted by BA
Exit row seats are located either next to or immediately behind the aircraft’s exit doors. If you choose an exit seat, you’ll be expected to assist us in opening the emergency doors in the unlikely event of an emergency evacuation.
For this reason, you must adhere to CAA safety requirements to be sat in this area.and I have had no issue selecting an exit row seat for my foster kid when 13 on BA
You must also:
Be over the age of 12 years old
Not be pregnant
Not have any accessibility needs or be Deaf/hard of hearing or Blind or have low vision
Be fit enough to operate an emergency exit door
Be able to understand printed or verbal instructions given in English, and be willing, as well as able, to assist in the case of an emergency evacuation.
As far as airline ticketing goes, child rates end after age 11; different airlines, different policies. Whetever the policy is, the cabin crew should be ensuring that the policy is adhered to