Isn't the fact that Tanaka is old 20 years after the war and that Shofixti captains are available so quickly (Is it actually just three months?) quite strong evidence of quick maturing and short life-span?
First off, we only have a minimum on his age, not a maximum. He could be substantially older than born just before the end of the war. He was able to take out an Intruder with his pea-shooter while limpeted, which is somewhat beyond normal capabilities.
Where does it say he's actually old, anyway? Hmm.
* Death 999 trawls the shofixti dialog...
The only references to him as old are:
#(dont_do_it)
Don't do it, old Warrior! You still have much to live for!
#(ON_SECOND_THOUGHT) shofi035.ogg
Wait! I have had a sudden realization!
If you are here, in such an amazing ship
that means there is yet hope to defeat the evil Ur-Quan pus-dogs!
A sudden exhilaration sweeps through my old body
I have changed my mind, Captain! I will not smash myself into an asteroid
instead, I will wait here, and when the time comes, Captain
when you have found the duty that only I can perform
I will be waiting here... eager to help you.
That end seems moderately well nailed down.
It isn't absolutely certain.
After being marooned in a tiny spaceship for 20 years, one might feel older than one is. And as for the Captain, well, everyone else who was a captain in the earlier war is now legitimately old.
And then, also, expectations of Shofixti life duration could normally be based on the time to sacrifice, not so much their actual potential lifespan.