By 'old', I mean questions older than 6 months.
If so, does anyone know the correlation between age of the question and Resurging?
This question, asked in 2011, was the most famous unanswered question on the Sci-Fi & Fantasy SE until one Kyle Hale finally found the solution (netting nearly 6000 rep from a single post, which has earned more points in bounties than any other answer on the entire Stack Exchange network).
The answer was posted late in 2015, more than four years after the question was asked. The massive upsurge in votes on both question and answer, due to the community's collective astonishment at the supposedly unanswerable question having been solved, was enough to send it into the Hot Network Questions despite its age, according to this comment:
I thought of the story as soon as I saw the post in hot network questions. Now I am lamenting not having seen it in the unanswered section. I still think about that story whenever I see two fives on a clock. – Engineer Toast Oct 23 '15 at 19:47
Per How do the “arbitrary hotness points” work on the new Stack Exchange home page?, this is the formula for HNQ:
[(MIN(AnswerCount, 10) * QScore) / 5 + AnswerScore] * Site Factor -------------------------------------------------–––––––––––––––- MAX(QAgeInHours + 1, 6) ^ 1.4
A post that is 6 months old would have a denominator of
((182.5 * 24) + 1) ^ 1.4 = 125,373.
Typically, the number of "arbitrary hotness points" needed to make the HNQ list is in the vicinity of 7–10. Right now the lowest post has 8.574:
Thus, in order for a 6-month-old question to get on the HNQ list, it would need to have a numerator of around 1,000,000.
The most important parts of the numerator are the AnswerScore (total score of all answers) and the Site Factor. On most sites, based on my own calculations, the site factor is around 40 (it's lower than that on Stack Overflow).
That means that if that question had a hundred answers, and they averaged a hundred upvotes each, the numerator would be around 400,000, still well below what would be necessary to get on the list.
TL;DR: It's basically impossible for a question that is several months old to get on the HNQ list.