Is there anything that can be done about old questions that have a valid answer that has not been accepted? For example, How many Mythos cards can be played during the investigator's turns?
This question was asked back in 2011, was answered, with an appropriate reference to the rules but has not been marked as 'accepted'
Could such answers be voted on by certain members with a high enough reputation so that they become accepted?
No, there's nothing. The very same idea was requested in 2009 (Would it be possible to have a "community accepted" feature?) and declined for reasons that are still current:
The check mark is simply a feature for the asker saying which answer might help them. Our voting is a way to rate which answers are good and useful.
Even if an answer is good and useful, it might still not have actually solved the asker's issue. The best answer to their question might be a crap one, or it might look good to us, but it still hasn't solved the user's issues, or it explains it in terms incomprehensible to them (e.g. the asker is a newbie to a game, and the answer is written exclusively to experts), or etc.
We shouldn't have an expectation that an asker must accept answers. That's why the accept rate feature got removed ages ago.
