Jackson Galaxy did a video on YouTube about this. My Siamese started yowling and meowing a lot after she turned 15 and we thought it was dementia. We tried keeping lights on at night for her to help, and kept everything as same as possible.
It had turned out she had an undiagnosed brain tumor that eventually took her. I would suggest trying out the dementia tips Jackson Galaxy suggests, and further testing with your vet to address whatever you can. Sometimes the yowling is your cat trying to tell you they need medical help.
My personal experience, as long as your baby is eating, drinking, using the bathroom, and has an okay quality of life overall (do they enjoy playing, spending time with you, etc), they can have a good few years left. Sorry you are going through this, it’s really hard to watch our baby’s health decline as they age :(