The Stewards of Gondor & their heirs by daughterofshadows

I initially started this investigation as part of my research for my Mereth Aderthad presentation. I always thought that Boromir was rather old for someone who was still unmarried and childless at the beginning of the Fellowship of the Ring.
So I decided to find out whether that was true when compared to the Ruling Stewards who came before him.

Methodology
I was more interested in the age the Stewards married, but Tolkien being Tolkien, the only Steward for whom we have a recorded date for that is Denethor II (who coincidentally is also the only one whose wife was given a name with the exception of Faramir, who was not considered for this exercise given that he is younger than Boromir.)
Instead, I settled on using the age at which their successor was born as a benchmark. Consequently, the marriage age is likely at least one or two years lower than the numbers given here.
All birth and death dates are taken from Tolkien Gateway, since I didn't have my books handy.

There is a total of twenty-six ruling stewards from Mardil to Denethor II.
Of these, two die childless (Dior {T.A. 2328 - 2435} & Ecthelion I {T.A. 2600 - 2698}). I will discuss them in more detail below.
Túrin I (T.A. 2165 - 2278) was disregarded, since he married twice. He had multiple daughters, who we know nothing about, but his only son and heir, Hador, was only born when he was already 80.

Results
I calculated the age a Steward was when their heir was born and sorted them according to the following age intervals.

As you can see, they are most likely to be in the 31-40 range, with 31 appearing 3 times and 35, 38 and 39 appearing twice each.
Based on twenty-three Stewards, the average age at which their heir was born, was 37.5, with a median of 38.
However, from those twenty-three, four have other children confirmed in the text.
Those four Stewards are Barahir (T.A. 2290 - 2412, one daughter, Rían), Denethor I (T.A. 2375 - 2477, two older sons who were killed when Uruks overran Ithilien), Belecthor I (T.A. 2545 - 2655, one daughter, Morwen) & Ecthelion II (T.A. 2886 - 2984, two older daughters).

I removed them and recalculated the average and median, but the changes were insignificant. (average age of 37.6 instead of 37.5 years, with no change to the median age of 38).

The effects of war & the lifespan of the Dúnedain
Aside from the basic ages, I was also curious to see whether there was a correlation between the age at which their heir was born and the Stewards' life expectancy (after all, the strength of the Númenorean blood diminished over time), as well as the amount of violent conflict during their lifetime.

I initially expected that I would perhaps see the earlier stewards having their children later due to their extended lifespan.
This holds true initially. We see a general downward trend in the graph (see below) beginning with the seventh Steward, Hador (T.A. 2245 - 2395).
This downward trend continues until the nineteenth Steward, Beren (T.A. 2655 - 2763). Both he and his son Beregond (T.A. 2700 - 2811) were busy fighting the Corsairs threatening Gondor during their lifetime, which likely is the reason why they had their heirs later than their predecessors.

Stewards usually inherited the rule later in their lives, long after their children were born, so to determine whether their lives were dominated by war or other conflicts, I found it more helpful to look at the experiences of their predecessor. Consequently, my graph indicates war and peace times based on the events occurring around the time they had their children, rather than during their reign.
With the exception of Beren and Beregond mentioned above, there appears to be little connection between the events occurring during the Stewards' lives and the timing of their heirs' births.
I was expecting to perhaps see that Stewards would have their children younger if they were embroiled in violent conflict to ensure the line was continued in case they fell on the battlefield, but that is not the case. Indeed, all Stewards in the under thirty category lived during peaceful times.
However, it also can't be said that Stewards necessarily had their children at an older age if they were involved in wars. We still see a majority hovering in the 31 - 40 range that is common throughout the entire line.

If anything, this exercise has made me extremely curious how Tolkien determined these dates when he put his timelines together, but I guess that will have to remain speculation.
Perhaps it is also a good reminder that we can't necessarily apply real world expectations on birth rates and the likes to a fictional universe.

Dior & Ecthelion I
Dior & Ecthelion I stand out as the two Stewards who died childless and were succeeded by a nephew and cousin respectively.
We have absolutely no information about their lives (at least as far as I was able to track down), so it is unclear whether they were unmarried, or whether their wives never conceived (or died or...).
Neither lived during violent times and died of old age, so it seems unlikely to me that they simply didn't get the chance to have children.

And that easily opens them up to queer interpretations!
I will, of course, argue for aromantic readings of both characters, as I am wont to do, but I think other angles are absolutely possible! Very rich soil for fanfiction XD

To me, they are especially interesting, because of the potential influence they have on Boromir and his own decision to remain unmarried and childless.
And on that note, let's talk about him.

Boromir
I set out on this research, because I wanted to see if my headcanon, that Boromir is uncharacteristically old for an unmarried Steward heir, is supported by maths.
The answer, just like the counsel you receive from elves, is both yes and no.
Yes, he is older than the average age of 37.5 years, but not significantly so.
Of the twenty-three Stewards I considered, fifteen had their heir under the age of forty, but notably, neither Denethor II nor his father, Ecthelion II, did. Ecthelion II may very well have had his first daughter under 40, given that he was only 44 when Denethor was born, but we have no dates to confirm this.
So Boromir isn't as out of the ordinary as I thought he was.

However, I don't think any of the things I discovered during this exercise undermine an aromantic reading of Boromir.
It is not a stretch to assume that the favouritism Denethor shows for Boromir extends to the point where he wants his line to continue through him rather than Faramir, and consequently that he might have pushed him towards at least considering marriage prior to his leaving for Rivendell.
Especially since Boromir regularly put himself at risk on the frontlines.
So it remains strange that he is unmarried and childless.

Equally, we have a precedent for childless Stewards. It could be interesting to explore a Boromir who styles himself after Dior and Ecthelion I, who expects his heir to be of Faramir's line, rather than his own, and who perhaps does so, because he has no interest in engaging in a relationship, in love.

And that remains very aromantic, doesn't it?