You would need to prove it was actually made by a minor.
Under EU rules - if they still apply in UK - any online purchase can be cancelled withing 14 days:
https://europa.eu/youreurope/citizens/consumers/shopping/guarantees-returns/index_en.htm
But it doesn't apply if you bought from a private individual - which may be this case. Probably the seller was happy to have offloaded to some stupid the stupid truck eventually, and doesn't want to wait for another one.
That's one reason not to enable any auto-payment features - I prefer I'm always required to enter credentials for purchases. If there's some form of 2FA, the better. Even for small purchases.
Biting the hand that feeds IT
