It could a lot of things other than old age, Johne's being one of them. If she is not used to eating concentrates you may struggle to get her to eat "new" stuff, but I would try some soaked sugar beet shreds, grass nuts, readigrass - do you have a treat that they normally like?
I have had good success with getting a couple of my goats that are struggling to eat post kidding, with a propylene glycol drench that they actually suck off the syringe - it is called "Selekt", and has additional stuff in it to encourage them to eat again.
If her teeth are still ok (I have a 10yo Shetland ewe with a full mouth), then willow branches, or ther young branches (soft wood) as well as rosebay willowherb will be liked.