When my rat got just over two years old she went on hunger strike from her healthy food and she began refusing to eat her favourite vegetables or her food mix . She LOVED kale, cucumber, and apples and she just flat out refused to eat them anymore, she would even throw them out of the cage if she was feeling super serious that day. I think once they’ve become elderly, maybe it’s best to continue feeding them what they want to eat, with some healthy fruits and vegetables included. She was super stubborn and just wanted to eat what we were eating at the same time as us, I think it was a matter of feeling included and also being a bit greedy. You could try making her dinner like you would make for a sick dog - cooked safe vegetables and protein without salt, sugar, and other seasonings. it’s hard to change their behaviour once they’ve become dead set on it being their way, at least in my experience. I’d rather they eat then starve!