There aren't that many that seem readily apparent, but there are a few.
Cranky
Dobie: Definitely the most elderly-looking villager. He'd be a perfect fit for the island, I think.
Jock
Billy: Another of the obvious ones, he definitely has that old man look.
Peppy
Audie: Not immediately obvious from her actual character model, but I think she would fit extremely well because, rumor has it that she's named after an 88-year old woman named Audrey Buchanan who logged nearly 4000 hours into Animal Crossing New Leaf.
Smug
Lionel: His mane is all white, and with his military uniform he gives the impression of a retired general or something.
There are two additional ones, but a bit more shaky.
Beardo: This one's a bit of a stretch, but his facial hair and balding hairstyle give him a mature look. This also extends to his house's interior design.
Chops: Same sort of reason as Beardo, both in terms of character design and his house's interior design.
Snooty
Cashmere: I just look at her and get grandmother vibes.
Claudia: Maybe it's just me, but her design give the impression of an older woman too.
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There are some that I think could fit, but don't really give off the elderly vibe outright. Mostly, they're just references to other time periods, but some of them just didn't scream elderly like the ones listed above.
Cranky
Not a whole lot to say, they just sort of look like they could potentially fit the part by looking more mature.
Gaston
Limberg
Monty
Walt
Lazy
Lazy honestly isn't the best personality for this. Not only do not many of the designs jump out at me as looking particularly elderly, but their dialogue doesn't seem like it would fit the part too well. But, regardless, the ones I came up with are...
Lucky: I mean, he's more or less a mummy, which tend (though not always) to be fairly old. Maybe an out of left field choice, but I could see it.
Clay: Another stretch, but considering he's based on a Dogū, and those were crafted so long ago, he sort of fits if we follow the same criteria as I just provided for Lucky.
Normal
Coco: Since she's based on a Haniwa funerary doll, which is the same inspiration as the gyroids. They were used between the 3rd and 6th centuries, so that was a long while ago as well.
Snooty
Ankha: Ancient Egyptian-inspired character, seems like she'd be a good fit if one follows the same criteria.
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That's about it. Despite Sisterly villagers supposedly being meant to represent "older sisters", none of them really fit any of my metrics.
Oh, one last thing, I really like Xeleron's suggestion of having Raddle there as a caretaker.