This is a BT customer to customer forum, it’s not necessarily a place to question ‘BT’, although BT employees are the forum moderators.
AFAIK, you can pay for BTWIFI access by credit or debit card ( presumably both need the card holder to be at least 18 ), direct debit (presumably the bank has done due diligence if they allow under 18 to set up DD, )or you can buy a voucher/vouchers ( so no difference to some PAYG mobile top ups ).
As far as someone else being ‘responsible’ for the under 18’s finances in this case, I suppose that would depend on the T&C’s of the bank account the under 18 has, payment being made from that account , and if an adult had to act as a guarantor for the bank account, is it an assumption or fact that the 12 month contract for BTWIFI is 12 direct debits from the under 18’s bank account ?.
Arbitrarily stopping the DD or removing funds from the account so the DD ‘fails’ may involve the bank checking their T&C’s and possibly involving a guarantor, so best to check that before acting.
If your son is accessing the same home hub as they would normally do ‘for free’ , just via BTWIFI, then there are other ways to block his devices , if another BTWIFI signal is available then restricting access is much harder, and the ingenuity of people his age normally find a work around anyway.
You suggest some sort of due diligence failure on BT’s part, internet access isn’t like alcohol or cigarettes and restricted to over 18’s
