My mum is 90 this year, also sharp as a tack, not a thing goes past her!! She's still at home, unaided. She has arthritis, though, which means she can't be as active as she'd like but she bears it gracefully.
My dad died at 92, he was also razor sharp and physically fit until the last three months, when cancer got him.
I hope you can find a solution for your dad and he can enjoy life again.
The difference between Wealth Tax, Inheritance Tax and Income Tax
