system
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human
[Star Trek] Given their life span why isn't Starfleet full of Aliens in higher command?

While most species ,in Starfleet, have life spans comparable to humans there are a few that can live a few centuries given a normal life span.  So one would assume that officers of some species would eventually get to a point where they couldn't be promoted further and would just stay in the higher commands.  And since you would only need so many Rear Admirals, Vice Admirals, Admirals, etc one might think most of them would be from species with very long life spans.  Is this the case and it isn't seen much in the shows?  Or is there another explanation?
gpt
It seems to me that a lot of Vulcans and other prominent species in the Federation tend to be opposed to the idea of spending their entire lives on one task. Because they live longer, they're far more likely to be able to start over or have a second career. Take Tuvok, for example--he was in Starfleet in the late 2200s, serving with Sulu on the Excelsior, until he decided he just didn't feel like doing that any more and left. Sixty or seventy years later, he changed his mind, ended up back in Starfleet, and served aboard Voyager. The same can be said for Spock, who left Starfleet to become an ambassador and eventually moved on to focus his efforts towards the Romulan/Vulcan Reunification movement. Worf did something similar, by most accounts, and withdrew from Starfleet to help rebuild the Klingon society in the wake of the Dominion War.

So from just those few examples, we can say that this is pretty typical of the longer-lived species. It's not like it's a hard rule or anything, though; we've seen plenty of Vulcan admirals ("The First Duty" comes to mind). There's also the fact that most Federation member worlds still do maintain some fleets of their own, which seem like a logical choice for experienced alien officers. What would you rather be: just another paper-pusher Admiral in Starfleet, or *The* Admiral of the Andorian Guard?