I was wondering what the average lifespan of a human (not in a red shirt!) is in the 23rd Century (TOS)?
As per a quick search on Memory Alpha:
The average Human life span had gradually increased during their history. the average life spans during the 22nd century was about 100 years (ENT: "Observer Effect"). This average age was still roughly the same during the 2250, but had risen to 120 by the mid-24th century. However, at some point in history the average lifespan for Humans was only 35, and by 1999 it had become higher than a millennium earlier. (ENT: "Similitude", VOY: "11:59"). Leonard McCoy had by 2364 reached the age of 137. (TNG: "Encounter at Farpoint").
So the average human lifespan had increased from what we currently have today by the 23rd century. For more information on other specie's lifespans, click the link I have provided to Memory Alpha.