This two and a half year old hen isn't old. Old age isn't what's wrong with her.
Look at where she came from. You say she's the last of her flock, the others having died. That's a clue that whatever killed those other hens may be working it's deadly way on this hen. If they were the same age as Penny, there's a possibility a pathogen in play and they didn't die of egg related issues after all.
You might try an antibiotic, but you will need to see a vet, and if it's a virus, it will do no good. If she dies, you might be able to get a necropsy and find out if exposure to her has infected your other hens with something.
Long ago, when I got my very first two adopted hens, around three years of age that I began my flock with, one died during the night soon after. I got a replacement from the same flock and everything was fine. But I had mysterious sudden deaths over the years. I've since found out that this isn't the normal way of flocks.
Finally, after a cockerel sickened and died at just eleven months, I got a necropsy and learned my flock carries the lymphotic leucosis virus, and it was likely brought into my flock with those very first adopted hens.
Not saying this is what's wrong with Penny, but it's a possibility.
