Hi
In my own - ample experience - it all comes down to a) whether a bereaved older sow is open to a new companionship or not (by far not all are) and b) personality compatibility and mutual liking.
I have adopted plenty of non-dominant and lovel natured neutered boars of all ages between 5 months to 5 years who were a total fail upon introduction and have ended up a couple of times with two anti-social sows bonding over their shared outlook.
My highly fear-aggressive Dryw now lives with two runty rescue-born babies I introduced shortly before her then ca. 8 years old 'daddy boar' Pioden passed from old age.
A few old ladies lived out their lives with next door company through the bars and by feuding with their neighbours - still social stimulation of sorts...
Unfortunately, you can never predict the dynamics between two personalities until they meet face to face. If you have to get a piggy on spec and cannot speed date them before you bring the newbie home then you will always have to have a plan B with a divided or adjoining cage for company through the bars allowing full level interaction (scent/pheromones, body language and vocalising) at all times.
If you have the space, getting a couple of baby sow companions for a tricky older sow may sometimes be the best way forward. She may reject a single baby but two can do the trick. And if there is a rejection, then you would at least end up with a bonded young pair that should last you a goodly while and which you could introduce to a neutered boar while they still have the biological urges pushing them if you wish to.
PS: My 5 years old Tesni 'Sun-warmed' fell head over heels for 5 months old Gethin 'Dusky' after her daughter Tegan 'Little Beauty' (the surprise baby in my avatar picture on the left) comprehensively dumped him 2 hours into the bonding. Tesni and Gethin were later joined by her widowed litter sister Ffwlbri 'Moonlight'. I adopted baby Carys 'Love' after Tesni's death aged 7. Ffwlbri lived on for another year, not least because of the renewed zest of life lovely-natured Carys brought her.
Little Carys is now 6 years old and very arthritic; she has sadly lost both her female best friend and the two husboars since Gethin died aged 6 (two of those at a younger age to cancer and one older widowed boar).
Carys now lives in a widows group (none of them very dominant) with a dominant younger boar called Brangwyn 'White Raven' who is thankfully not oversexed and who has calmed down a lot since reaching adulthood a few months ago.
Gethin with Tegan just before Tegan sent him packing in the bonding pen (March 2016)
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Tegan's mum Tesni with her 'toyboy' (their's was a trule love at first sniff match) (March 2016)
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Gethin with old sisters Tesni and Ffwlbri in early 2018
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Meeting baby Carys and Ffwlbri in September 2018
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Carys with her new girlfriend Tegeirian 'Orchid' to help her over several years older Gethin's loss (June 2020)
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Widowed 'husboar' 2 years old Tudur (Tudor) 'Tribe Leader' with Heilin 'Generous' and Carys and Tegeirian in December 2022
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Carys with Brangwyn and fellow widowed sows. (Heilin has sadly died a few weeks ago, aged 5, and Esme now lives next door with a more dominant younger widow who didn't gel with the group over who should be the First Lady)
Spring 2024
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This here is another life story:
Baby Dryw 'Wren' with 8 years old Hedydd 'Skylark' (just days before her death) and then ca. 5 years old Pioden 'Magpie' in 2019.
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2 years old Dryw being bonded with 2 baby girls, Llinos 'Linnet' and Lleian 'Smew', not long before Pioden's passing in November 2021 - the Pandemic prevented an earlier adoption
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Since Dryw is not accepting any other piggies, they are still together as a trio. Dryw is no 5 years old and the two tiny cousins (600g on a good day) have just celebrated their 3rd birthday. I hope that my little firecrackers will manage to bond with Brangwyn after Dryw's death since they are only a year old then him. But any bets are off!
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I hope that this gives you a bit of an idea? I assure that I have had plenty of bonding fails trying to match up bereaved older sows of mine until finding a new solution for most of them.