Google's support page says otherwise
https://support.google.com/youtube/answer/10070779?hl=en-GB#...
https://support.google.com/youtube/answer/10070779?hl=en-GB#...
Credit cards in the EU are not very prominent as a payment method, mostly for paying USA companies.
You don't need anything. At all. According to Google, in case they suspect you might be lying about your age, they _might_ require age verification. This can be done with a credit card or an ID card.
So no, you don't need a credit card to watch age-restricted videos. You don't need a debit card either. You probably don't even need an ID card or anything at all.