WOW~ 20 years old! Bless the 'ol gal/guy!
At that age, there is sure to be something going on that is causing him/her to lose appetite and lose weight. I would recommend taking your cat to a vet. to have some bloodwork done. Could be he/she needs some medicine that might improve the appetite as well as the underlying issue.
Good Luck!
quite often an older cat will stop eating. There are a number of ways you can treat this 1) put a little concentrated vitamin past on top of their paw and they will lick it off (you can buy this from your vet or pet store 2) Cuddle you cat like a baby and use a syringe (without the needle of course) and use the small animal milk that you buy from the vets or pet store to feed kittens and small animals and mix it up as directed and syringe into the corner of your cats mouth 4 times a day about 40 mills each time. 3) warm up some cooked chicken and offer it two him/her normally cat food warmed up slightly will help them smell the food better as with age they sometimes loose their smell. bestest