Regarding the issue of lifespan-long terms, an attempt to resolve the issue with headcanon:
I think the issue can be viewed through the egalitarian-authoritarian prism. In an egalitarian meritocracy, fairness would be a major concern and indeed an important component of what their meritocracy is about. For them, lifespan-long terms that exclude candidates would be against what they stand for.
On the other hand, an authoritarian meritocracy would be less concerned about fairness - the important thing is what is good for the state; that ruler (s)elections are done in such a way that no candidate is disregarded due to societal privileges or irrational prejudice. That one or two generations of potential candidates will be overlooked due to bad luck may be unfortunate, but since those generations' best candidates are not expected to be significantly better than any other generation, it is not really a big loss for the state. It is a price worth paying for the benefits of long rule, or alternatively: being born in a "middle generation" would be a negative merit when considering the objective interests of the system. The authoritarian meritocratic concern would, after all, primarily be about what serves the system best.
(If they are superstitious, they may even consider the "middle generations" to be inherently more unlucky than other generations, and that could be sufficient reason to not allow those generations to participate in the selection process. A potential ruler with bad luck could be seen as very, very bad thing in a Fanatic Spiritualist meritocracy.)
It is worth noting here that this difference between Egalitarian and Authoritarian meritocracy already would be reflected in the game mechanics, as egalitarians get a much bigger total specialist bonus; Egalitarian meritocracies would always be better than Authoritarian meritocracies at finding and promoting talent, precisely because of their concern for fairness and equality of opportunity, not just equality in opportunity.
---
History also has a yet unmentioned aspect to contribute with on this topic, namely that age itself is often used as a term-regulating factor in lifespan-long elections. This happens frequently in the elective monarchy of the Vatican, where old candidates get elected when the cardinals do not feel comfortable electing any one candidate for a long period. According to a quick websearch, since 1750 the median length of a papal reign has been 13 years; the longest papal reign during that time was just over 31 years (also the second longest ever). A short life expectancy makes it likely that a new election will be held in a not too distant future. Similarly, if there is a "young" candidate that is considered to be really strong, they can get elected with the expectation that they will serve long and well. While I am not a scholar on Vatican elections, I would not be surprised if there is a correlation between the age and perceived merits of a candidate at the time of election.
(Also, while many popes historically have served long past their peak, the last pope Benedict XVI chose to abdicate on February 28th 2013 due to his declining health. However, since the catholic church already has Philosopher King and Exalted Priesthood, the only way it would have a third slot for Meritocracy is if they have discovered the Galactic Administration tech - but how could they possibly have done that?)
It's possible to choose rulers based on merit without being a meritocracy. Skipping over generations of potential rulers, for whether the reason, I'd argue excludes that empire from being a meritocracy since things other than skill and talent are considered.
The way I would define the Egalitarian vs Authoritarian split on meritocracy is whether being better at your job makes you worth more as a person. For the authoritarians, someone who is highly skilled would be highly placed in society, possibly placed in an exclusive housing district and their "vote"/influence/opinion given more weight. Where as for egalitarians, sure a great chef might be paid more and be more popular, but it's not like they are a better person or get an extra vote.
- 1