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In my story titled La Maison Sapiens, there is a HUGE nuclear family composed of the father, the mother, and their seventeen biological children.

To be exact, the dyadic cisgender man and the dyadic cisgender woman, who are the starting members of this nuclear family, were respectively 18 years and 10 months, and 19 years old and 6 months when their first biological child was born. When their second child was born, the father was 22 years and 3 months, and the mother was 22 years old and 11 months. When their third child was born, the father was 26 years old and 5 months, and the mother was 27 years old and 1 month. When their first pair of twins were born, the father was 29 years and 4 months old, and the mother was 30 years and 0 month old. When their second pair of twins were born, the father was 34 years and 1 month old, and the mother was 34 years and 10 months old. When their first set of triplets were born, the father was 39 years and 6 months old, and the mother was 40 years and 2 months old. When their second set of triplets were born, the father was 44 years and 9 months old, and the mother was 45 years and 5 months old. Finally, their last biological children, who are quadruplets, were born when the father was 48 years and 11 months old, and the mother was 49 years and 7 months old.

So, the question is: is it realistic for anatomically modern human siblings to have three decades of age gap?

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    $\begingroup$ This is a biology SE question, rather than worldbuilding. But yes, you can have siblings even further apart if menopause occurs late and reproduction starts early. Mid-fifties is the upper average limit, with IVF extending that. en.wikipedia.org/wiki/…. $\endgroup$
    – DWKraus
    Aug 16 at 1:55
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    $\begingroup$ As written, this is not a Worldbuilding question. A Worldbuilding question would be 'Is it plausible ?' and there's not really much of a difference between 'modern' humans and 'non-modern' humans from this perspective. And, if it's plausible and the story requires it, who cares how realistic it is? Winning the lottery isn't realistic, but that doesn't stop a story from using it as a plot element. $\endgroup$
    – Halfthawed
    Aug 16 at 2:46
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    $\begingroup$ (a) Modern genderism has nothing at all to do with the fact that the human species requires a sperm donor (the male of the species) and an egg carrier (the female of the species) to propagate. What they call themselves (or what we call them) has nothing to do with your question. (b) VTC as not about worldbuilding. This is a Biology question. $\endgroup$
    – JBH
    Aug 16 at 4:02
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    $\begingroup$ If re-asking this on biology, please ensure to show your prior research into the subject and where you got stuck. Also, use scientific (biological) terms rather than ones from the sociology crowd. $\endgroup$ Aug 16 at 4:07
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    $\begingroup$ Try putting "biggest sibling age difference" into a search engine and look at the results. Much less time required than to type your question. $\endgroup$ Aug 16 at 7:45

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Covering all the details you've given, it's not likely to happen naturally. Triplets are rare enough already, and quadruplets are nearly unheard of without the use of fertility drugs. Fertility also drops with age, which is another blow both against that detail and against the idea in general. If you're willing to use in-vitro fertilization and/or fertility drugs, this might be possible, but trying to call this a natural occurrence would certainly strain my suspension of disbelief.

However, your general premise is not impossible, even without any artificial assistance. A quick Wikipedia check tells me that menopause is typically around 47 to 54 years old, which is within your range. Fertility is typically lower in the years preceding menopause, and I believe you're more likely to have a high-risk pregnancy with mothers of that age, but those merely make bearing children at that age rarer and/or inadvisable, not impossible.

More to the point, puberty sets in rather sooner than 19 years old. By modern Western standards, a pregnancy at 14 or even 16 is alarming, but such things happen. Exploiting that gives you a few extra years of wiggle room to work with. If you want to push that range down farther, there are known instances of people becoming mothers before most people hit puberty, albeit trying to match the youngest cases would not be wise as a practical matter; readers would undoubtedly be horrified if you tried to use the case of a six-year-old having a son.

Between these, then, you have a practical range of about 35 years (15 to 50) to work with; you can push it farther if you really have to, but this would be your approximate average in nature. Three decades is testing those limits, but it's entirely possible; dig around through enough records, and I'm sure that there are some real-life cases of such gaps between siblings. If you're willing to add in-vitro fertilization, women are still capable of giving birth even after menopause: this isn't necessarily a good idea due to the medical hazards, but it has been done, and for the purposes of your question it would extend your range further.

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  • $\begingroup$ stanfordchildrens.org/en/topic/… for more info. $\endgroup$ Aug 16 at 2:03
  • $\begingroup$ This is really more a biology question. For the kind of answer, I'd add some links and references. $\endgroup$
    – DWKraus
    Aug 16 at 4:34
  • $\begingroup$ @DWKraus My reference was 5 minutes of browsing Wikipedia looking for information. Still, probably a good idea to link in the pages in question. $\endgroup$
    – Palarran
    Aug 16 at 9:59

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