If I buy a Psn card at the petrol station to buy a game through the Psn store, do I still have to confirm my age at ps4?
You have your PSN account, if you are over 18 you have all permissions unless your PSN account is registered with a minor birth date.
Otherwise you only need proof of age when paying for the card.
If you indicated in your PSN account that you were over 18, then not (at least that's the case with Xbox).
Otherwise you have to somehow confirm that the game is being bought by an over 18 year old (if it has an FSK of 18) (e.g. Using the bank account of an adult).
Correct me if I'm wrong.
No, if you get the card everything is fine. With Psn cards you don't have to confirm your old one. With Paysafe cards, however, you must provide your ID (one over 18 years old)