Question: I was brainstorming with a bunch of friends about a story with more than one sapient species on a planet. The idea was to have different lifespans for each species as part of their differences.

I thought about having the usual humans with a standard 70 years lifespan and other species with ten (10) or a hundred (100) times longer lifespans but then the idea of a species with a shorter lifespan came up.

These species are supposed to be able to create a fully functional civilization with culture and some technology and given that most cultural and intellectual endeavors and most skills, blacksmithing, for example, take a lot of time to developed I worried that if we made their average lifespan too short they would not be able to form any sort of civilization.

My question is: what do you think is the minimal lifespan that would allow a species to develop a functional human-like civilization? By this I mean they have some sort of culture like music and religion and technology, like metalworking or masonry, academic pursuits like mathematics and science, and functioning governments and economies et cetera.

Also these species are also as smart as humans, if they are super geniuses then the question is pointless. And lack magic or superpowers that could let them transplant knowledge from other individuals into themselves. Basically they cannot cheat to learn stuff

Answer: 
**3 years. But their ancestors lived a lot longer.**

In the remote past your species was much like humans, and developed science and arts (and war, and medicine) as we did. These skills were usually passed in the family.

Individuals who were better adapted to their skill had access to more resources and so had better genetic fitness and reproduced more. Evolution can happen faster with shorter generation times and so time to reproductive maturity grew shorter. These things grew up and had kids fast. A consequent is that they died fast too.

The result now is a caste system, whereby strains of this species are adapted for the skills they perform, like ant castes.

[![ant castes](https://i.stack.imgur.com/BPHhP.jpg)](https://i.stack.imgur.com/BPHhP.jpg)

This is true for the artisans and craftsmen, the leader / scientists, and especially the warriors who are capable of frighteningly fast reproduction when their kind is needed.

Selective pressure on the physicians went the other way and the doctors are functionally immortal.

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Concepts lifted liberally from [The Mote in God's Eye](https://en.wikipedia.org/wiki/The_Mote_in_God%27s_Eye). If you like high science fiction do not read the wikipedia article (spoilers!); just order the book. It should be required reading for would-be worldbuilders.
My answer is a modification of L.Dutch's, but I see other answers making similiar assumptions about the parents alone raising children.

The first and third generation do not need to overlap. Rather, lifespans need to be long enough that the oldest children *in the extended family* can care for the youngest while the parents resume their civilization-building activities. If older generations overlap it is undeniably a bonus, but not strictly necessary for the *minimal* lifespan.

From the Paleolithic to Bronze and Iron Ages, human life expectancy hovered around 30 years, ±3. Menarche began, as it does today, between the ages of 7 and 13 or 25-50% into the lifespan. This gives us a narrow window of just a few years in which children are physically capable of helping to raise younger children before they set off to start families of their own. Assuming age 10 for the first successful pregnancy and one successful birth every two years, by age 16 the first child (age 6) will be capable of basic assistance with the third; by age 18 the first and second child will be capable of minding the third and fourth for most of the day.

So, that leaves us with two key variables to work with: the age an individual is physically mature enough to help care for others, but not have children of its own, and the immature share of the lifespan prior to that. In humans I'd argue at a minimum the latter is 5 years, and the former (based on the above) is 5–8 years. Combined this is 33%+ of the lifespan, implying by the 10th child (using the above assumption) of the first generation, the second generation is already several children along itself, at or near the point they can begin to help their aunts and uncles raise their cousins and siblings.

It would seem trivial then to say the solution is to minimize the immature stage and maximize the mature but pre-childbearing stage, so long as the two combined do not exceed the length of the third, childbearing, stage. If you hold to the human-like constraints, this puts the minimum lifespan around 20, with little room to compensate for death and disease.