I would agree with a change of food - my cat is the same age as yours and now eats more 'wet' food than dry. He gets through 2 pouches and a saucer of biscuits per day and hasn't lost weight - it's just that the balance between wet and dry food has changed.
If you don't want to change from her usual dry food you could always put warm gravy or boiled water on the biscuits to soften them, allow it to cool before serving though.
My cat lived to 19 but he was a bag of bones. He ate loads-the same amount he always did ( the soft pouches) but just didn't put on any weight. The neighbours thought he was a skinny stray but the vet said he was just very old.
I work with cats and the old ones are usually bony and just 'slower' .I think my dog had dementia, she often looked very confused in the last years as if she didn't know what or where she was. Miss them.
Has the vet given her a blood check? Our Rover lost weight due to his kidney problems (he was fine later on medication) and the cat next door is painfully thin because he has thyroid problems. Both conditions are common in older cats, I hope the vet is checking for them.