Milady is a Herdwick, one of my favourite breeds, and Herdy's go white with age. She's almost snow white, so I can only guess she's probably in her early teens. She came off the fell, and since Herdy's are hefted - bonded to their particular patch - she could never go back as wherever she was put she'd try to make her way back to her bit of the fell.
One of the biggest issues with ageing sheep is teeth. Obviously being a grazing animal their teeth get some stick over the course of their lives and they can eventually become broken mouthed - that is unable to eat. That's why I ended up having Milady's sister put to sleep as she couldn't eat properly. They do lose teeth as they get older, you can particularly see this with the front teeth, but can generally still graze okay. I think problems with the molars are more serious as these are in use for chewing the cud, and unfortunately there's little you can do if it gets really bad and they can't chew properly.
Milady is very up and down where food is concerned. We try and make sure she gets extra, but she has a tendency to go off food from time to time. Offering novel foodstuffs can be a way of tempting them to eat, things like cleavers, cow parsley, we cut some longer grass, and trimmings from willow are also particularly good. I have heard they really love ivy but we don't have that around here unfortunately.
Arthritis is one of the easier things to deal with in an older sheep as your vet can dispense a painkiller that can be easily mixed with food. The only issue with this is separating the sheep to make sure the one you want actually eats it! There's nothing like the allure of what someone else has got if you're a sheep.
Knowing your sheep really is key, it's only then you can spot potential problems early, and also ensure they're having quality of life. I know every day I have Milady sheep I'm grateful and try to do my best by her.