It’s not strictly true that a British Airways passenger under the age of 18 cannot be a member of the Executive Club.
The rules stipulate that a minor cannot be a ‘primary member’ of the programme, and one of the several reasons for this is to do with access to UK lounges where alcohol is available for self-service.
However, it is perfectly possible for an under 18 year-old to be a member of the Executive Club as long as they are part of a Household Account formed and administered by the ‘primary member’ who is over the age of 18.
The minor is therefore considered to be under the care of the adult and, for the purposes of the programme, can earn BA Miles appropriate to the fare paid. Redemptions using Household-held miles can only be made by members over the age of majority, though they can of course do this to benefit the minor(s).
The full terms and conditions are here on the BA site:
– the relevant clause being 16.1.12