People are listing the obvious but missing the correct answer: mental illness causes suicide. The brain goes in old age, which doesn’t just mean cognitive impairment but personality changes, mood and behavior regulation difficulty. Older people may hallucinate, are typically delusional and paranoid, they have trouble controlling themselves including mood and excretory functioning. There are notable examples in the public view right now if you’ve never experienced it. That someone not in control of their own mind might kill themselves is obvious. There needn’t be rational reasons like those listed and I suspect there often aren’t.