I'll try to answer what I can; there may be others who have had more experience than me.
I would give them a cage with adequate space for 2 boars. There's no need to make it smaller because if they are arthritic, they'll naturally restrict themselves anyway, and you want enough space so that they don't crowd one another. Besides, frequent, gentle exercise is actually beneficial for arthritis.
Extra vitamin C is a good idea for elderly pigs. I think you want to ensure 50mg vit C daily for older pigs (if I remember correctly)? So basically add a 25mg supplement onto their daily vitamin C veggies.
My boar who lived to age 6 never got impacted, but I did make sure to check and clean his perineal sac about every 6 weeks or so (just to clean out debris and look for any evidence of impaction problems). I also had to clean his grease glands about every 6 weeks too, but I didn't have to do it more often as he got older or anything; he was just kind of a greasy pig his whole life.
A lot of whether or not a male is likely to have issues with impaction is related to exercise. Impaction generally occurs as a result of muscle tone loss in anal sphincter as the animal gets older. If they can stay reasonably fit and active throughout their lives (including as they grow older) they're less likely to have issues. This is another good reason to give them as much cage space as they need to run about if they ever get the urge to do so. As I said, my pig was very active during his daily floortime and had adequate cage space, and he remained active to his very last days and never had impaction issues.
I'm not sure what else to mention right now other than you should be prepared -- financially and logistically -- to potentially have to deal with medical issues. Just because they're older doesn't necessarily mean that they can't remain healthy, but certainly age does increase the chance that they may run into medical issues at some point.
Good luck, and thank you for considering the older fellows who need a home too!
