It is worth having it assessed by a doctor, but since dementia affects the brain which controls everything then that is the most likely cause. But lots of elderly people develop tremors in their hands or jaws, simply as a consequence of old age. Such tremors are often referred to as "benign" or "essential" which means they are harmless aside from the annoyance of them and that no specific disease can be identified as a cause. It's sometimes possible to treat them but that's usually not done unless the tremors are affecting quality of life, since as with all drugs the treatments carry with them their own potential problems.