How old an can an expired domain get without losing its value?

Let's say you find a decent expired domain that used to be the website of bank. It has some decent links, yet it was last active online in 2006, then it was closed and nothing was on it for 12 years.

Will the domain still be useful as a PBN even in this situation?

I don't know much about SEO . But, I think if its backlink profile is still good and the links are not broken or spammed then it should work.
Don't take my word for it. wait for the experienced guys to answer

It won't matter as long as the links don't drop. This is assuming that you've already done your due diligence for the domain.

I can't talk for others but I personally only use domains from the auctions. They stay down for much shorter period of time. I don't like expired domains which stay down for years.

from what little i know, i do not think they lose their value, as long as the majority of the links are still active. this would be a good question for @Nargil

I doubt that very old domains are useful, although some people claim, the older the better.

G can easily figure out how long a link points from A to B/Nirwana. And that's the nature of a backlink: giving a vote to the content of another site/page. If the vote is not valid for looong time, why should it give any credit? There are reasons for giving short-term credits like transferal of ownership of domain, etc. But after 10 years there should not be any value left and the corresponding backlink won't be considered anymore. I personally believe it is not a b/w decision, but a gradual descent. Ideally, go for fresh and crispy.

Let's got one step further - some PBNs rock (you link from your PBN to your money site), other do not. I think this is a matter of luck, but why?: G knows about active how webmasters deal with broken links. So they have a measure for outlink credibility. Sites linking in PBN domains with a lot of broken links won't give a lot of credit for obvious reasons. But if you are lucky, your PBN has backlinks from a credible site - the webmaster of the site did not note that the domain disappeared for a while, and so the link remains. Certainly, there are other factors as well.

How quickly might a PBN loose its power? Imagine X,Y,Z are linking to the PBN. Although X is credible, Y and Z made their homework and links to PBN disappear. G can easily figure out. Maybe not instantaneously, but in batch jobs which are carried out regularily. The removal rate might be pretty non-linear, so that even after shorter times, the guys W and Z already ruined the power of the PBN and make clear to G, that X should not be considered, although the link is still there.

To sum up: the older the pdn domain, the less is the propability that you gain power. Losses might sum up quickly.

It really does not matter. As long as the domain has active backlink profile, it will provide link juice. Compared to auction domains that are still partially indexed, it might take a little bit longer till they kick in, but they will kick in eventually. Time is irrelevant in this case.

Another logical thing is, that expired domain that has not been registered for years has not been registered for a reason and that its backlink profile will not be anything to kill yourself over with.

If the backlinks are there, that's all that matters. If the site was lying there with no site for many years, it might be deindexed, but usually it jumps back to index very quickly once you restore the site.