First, if your cat is not acting normally, go to a vet to make sure your pet is healthy. Especially if your cat is vomiting, has diarrhea, is eating more or eating less, and is urinating more or urinating less. Plus, see your vet annually for preventive care.
Keeping older cats groomed, hydrated (water in multiple places), providing good nutrition, exercise and brushing their teeth can help your cat as they age. These may be things you were doing all along with your cat but now they become an imperative. Plus, nutritional needs are changing.
Similar to humans, as cats age their senses can become diminished, hearing, vision, sense of smell and dental diseases. They can also get diseases such as heart, kidney, diabetes and arthritis. Check for lumps or sores and check out changes in their activity level if they are dramatic.
If your cat is not using the litter box - do they have arthritis or is it hard for them to get in it then you might need a box with lower sides. If they have a kidney disease, UTI, or diabetes and are urinating more, the litter box needs changing more frequently. After you have checked out why they are urinating more, placing more litter boxes around could also be helpful.