Are these Netflix prepaid cards over 18?

This one:

No, you can buy them under the age of 18.

But there are also films, series from 18 on Netflix. Isn't that really from 18?

This has nothing to do with the content on Netflix.

Look at their terms of use, there it is

Oh so ok.

Gladly.

It is seen differently everywhere. On the whole, however, a minor can buy a credit card. It also does not exceed the pocket money paragraph! However, it is a matter that is being discussed extensively because there are films and series that minors should not necessarily see…

Here you can read a little more: https://rp-online.de/panorama/fernsehen/netflix-wie-man-kinder-vor-nicht-jugendfreien-inhalten-schuetzt_aid-16703981

You can also buy one under 18