Even if you include debit cards, you're still wrong.

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.

You don't need to be of legal age to get a debit card, but you do for a credit card, hence why credit cards are often used as age verification.