Little old ladies - love 'em
Marches Farmer has some good advice in there.
First off FrancesV, how old are your 'old ladies' and what breed/s are they? My experience is limited to Primitives and females. When to decide they are 'old' and in need of special care does depend on their condition. I have another ewe who is 16, which many would think was ancient, but she certainly doesn't think so. She was mortified when I put her in with Blondie, my last remaining ancient - she bawled her head off til we let her out again and charged off down the field like a bat out of hell to get to her mates
I have only one really old girl now, Blondie a white Heb
who is either 18 or 19, but as she's not registered we don't know for sure. To look at she's a bit like a toast rack covered in wool, with a very dirty face from spending a lot of time with her head in a molasses bucket. However, she can still skip along with the best of them. She spends her time between sitting in her shelter, which faces the sun so can revel in that, surrounded by her pals the hens which love to sunbathe too, and being out grazing or at the hay heck. Ideally she should have a companion, but her chum died at Christmas. We let her in with the rest of the flock for a gossip whenever she wants to, but she soon gets bored and demands to come back to her own paddock.
She needs slightly longer grass than normal so she can pull it with her gums, and also eats ad lib hay, coarse mix, her licky bucket and of course Digestives. She still has all her molars but no front teeth at all - this is ideal for a sheep which has started to lose front teeth - once the molars start to drop out then that's the end.
We found it better for the really old girls to keep them in a separate small paddock, with the food just for themselves as they eat more slowly than younger sheep and tend to get pushed aside at the trough. Same with Digestives - they wouldn't get any if the rest were crowding round.
They definitely need a shelter as the wet is what gets them, so if they are in with younger sheep they won't be able to defend their part of the shelter and could end up standing out in the rain. The fleece tends to thin in old age, so they don't have such protection as a younger sheep.
We would never keep them indoors - they would hate it and would probably rather be dead
We concentrate on keeping these old ladies very contented, but if for some reason things start to go wrong, then that is when we call in the knackerman. Sometimes they just die in their sleep as Jezebel did, just a few months short of her 20th birthday. If you don't have time to spend with these old ladies and really make pets of them, then it's not fair to keep them going just because you can.
Of course they need all the usual treatments all sheep need such as foot care, shearing, worming, anti-fly-strike and so on.