ARE LIEUTENANTS REALLY THAT EXPENDABLE?

Empirical evidence suggests that the average life span of a military lieutenant during wartime in the olden days (1618-1918) oscillated between the ages of 18 and 28.
So that Thundercats quote is right, because the same average lifespan applies in fantasy worlds with professional armies.
What measure was a lieutenant? Just a subaltern officer, a chess piece of middling value (neither so high nor so low). They nearly never wrote history, they just served under colonels who, in turn, served high-ranking and famous military leaders like Gustavus Adolphus, Frederick the Great, or Arthur Wellesley.