The future society portrayed by Ayn Rand in Anthem is actually a society based on humankind's past. Essentially, the setting she creates is one of humankind before all the technological and ideological advances that had been made when Rand wrote the text in the twentieth century - essentially the Dark Ages placed in the future.
So - just like in the Dark Ages - humans do not live as long in Anthem's society as they do now because they do not have the advances in medicine, safety, diet, etc. that we have now.
In addition, there is something to be said about the mental health of the people living in a Dark Age. In chapter two, Equality 7-2521 describes his brethren: "There are Fraternity 2-5503, a quiet boy with wise, kind eyes, who cry suddenly, without reason, in the midst of day or night, and their body shakes with sobs they cannot explain. There are Solidarity 9-6347, who are a bright youth, without fear in the day; but they scream in their sleep...'Help us!' 'Help us!'" Any modern psychologist would say these men suffer from severe untreated depression, which would greatly shorten a person's life if left untreated.
Finally, at the age of forty, these people are forced to go to the Home of the Useless. "They sit in the sun in summer and they sit by the fire in winter." What person wouldn't take that life over the life of a workhorse? And once there, what person wouldn't slowly pass into death with nothing to do but remember a life of work and sadness?
that is a really good question i was wondering that too
the only thing i would be able to think of was that they work so hard everyday and dont sleep as much and arent happy. maybe they just dont have much to live for? their bodies just give up at a younger age