There are various reasons for this, as discussed in the above article. Other considerations for elderly cats include arthritis and cognitive dysfunction syndrome (CDS).
Arthritis is a degenerative disease that causes painful joints. Often, signs of arthritis in cats can be subtle. They might vocalize while using their litter box due to difficulty accessing it or discomfort from squatting.
To help them, provide a spacious litter box that has low sides so that they can climb in and out more easily. If you suspect your cat has arthritis, contact your veterinarian so that a management plan can be made to keep your cat comfortable.
With old age, cats can suffer from cognitive dysfunction syndrome (feline dementia). Increased vocalization, especially at night, is one of many signs that they might show. They might meow before using their litter box due to confusion and anxiety.
Other medical conditions need to be ruled out before a diagnosis of CDS can be made. Again, it is best to contact your veterinarian for further advice on how to proceed.