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: 
On Earth, there is one species which is known or believed (by its members) to be intelligent and to be civilized.

There are a number of other species of large brained mammals which might possibly be intelligent beings, though none of them are civilized.

If in the future ways to communicate with those species on equal terms are invented, it might be possible for humans to teach them how to develop advanced technology. In such a case the number of civilized species on Earth would rise from one to a higher number, perhaps as high as about a hundred.

I note that a majority of the potentially intelligent species on Earth are cetaceans, and they lack limbs useful for manipulating their environment, and also live in the sea where making fire to smelt metals would be a problem.

In Larry Niven's story "The Handicapped" a company sells artificial limbs to "Handicapped" species such as dolphins and Bandersnatchi, and a representative of the company arrives on the planet Down to investigate whether the sessile Grogs are intelligent and potential customers.

[https://archive.org/stream/Galaxy\_v26n02\_1967-12\_modified#page/n79/mode/2up[1]](https://archive.org/stream/Galaxy_v26n02_1967-12_modified#page/n79/mode/2up%5B1%5D)

Thus it seems likely that the various species of possibly intelligent beings on Earth are more likely to become members of human civilization instead of developing separate civilizations on their own.

And possibly that may be the pattern for habitable planets in real life, that a number of different species of intelligent beings evolve at the same time, and one of them develops civilization and the others eventually become part of that civilization.

Of course the unified civilization of all intelligent species which might hypothetically develop might later splinter into separate civilizations for each separate species.

In any case, it might be a good idea to investigate the life spaces of the various possibly intelligent, though not (yet) civilized, species on Earth to see how much variation in lifespan there is among potentially intelligent species on Earth, and perhaps use that as model for your fiction.
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.