Thanks, Michael 💖

Finally - I'm so slow - I think I understand. If you add a "that" to your "which", and change your "are" to "is", you might come to "That which is is old is not necessarily right" - which would be fine too, if a little old-fashioned or formal.

Yes. Being the oldest book doesn't necessarily mean it is right - this is all OK. You could expand that opening a little, perhaps: 'Just because it's the oldest book doesn't necessarily mean it is right' - but this is not better than your version. https://www.collinsdictionary.com/dictionary/english/just-because

Can I write it like this, Michael? "Being the oldest book doesn't necessarily mean it is right"

The statement is difficult to understand because it is ungrammatical. If I were to guess, I think it means that because something (or maybe someone) is old or traditional that doesn't mean that it's right. I hope this helps.