What's wrong with my hen? Is it old age?

[CozyDia]

I have a old White Leghorn hen that has been looking pretty unwell. About a month ago, she contracted a respiratory illness where she sneezed and gasped often. It didn't seem infectious as none in the flock got it. I treated her with VetRx and Nutridrench and hoped that with time it will get better and eventually it did. She stopped sneezing and gasping, but now after a week of being seemingly better she has started looking worse for wear.

Her feathers are very dirty despite being bathed not too long ago. There are black (I assume dirt?) stains on her head feathers that won't come out. Her belly is bald. Her tail feather is in tatters. Her once bright red comb is now dull and floppy. She pecks the oil gland on the base of her tail often. She's not as active and has been behaving questionably these past couple days especially when its time to turn in for the night (she causes disarray among the flock's usual sleep spots).

I can't figure out what's wrong other than perhaps old age? I don't think its ticks or mites since her bare skin is pristine. Her poops look normal like everyone else. There is no physical deformity/injury that I can discern. She still eats and walks around, but she does so very very very slow.

 

[Eggcessive]

Do you have any pictures? How old is she? Could she be molting? Does she lay eggs anymore? Does she dust bathe? Can you feel of her crop early in the morning before eating and drinking, to see if it has emptied overnight? Have you ever had a respiratory infection in your flock besides her in the past? Have you looked over the other hens for any parasites, mites, or lice? Try offering some wet chicken feed and cooked egg, and if you have pictures of droppings, those can be posted. We have all had old hens who kind of do their own thing, and I tend to let them be, but check to see if they have any specific problem. Worming her with Valbazen or SafeGuard might help if that is a problem.