I'm 45 and still have a grandfather alive (he's 101). So you're certainly right it could be a long stretch yet, potentially almost as long as I've been alive!
Last time I saw him he didn't remember me but he told me stories about the nursing home anyway, frequently repeating himself. When I had to leave he held my arm and looked at me, almost pleading with his eyes not to go, although he didn't push it. His days are spent sitting in that room watching the God awful TV. He can't focus on reading anymore, can't go for a walk (wheelchair bound since about 95), can't remember his family or his past so they've almost entirely stopped visiting, can't control his bowels anymore, and his only regular human contact are with the nurses that help clean him up which he is ashamed about, and visitors for an hour maybe once a week.
This is a man with a huge extended family, who was happily married for over 60 years, very widely respected and influential for most of his life. Almost a model life. Someone I have admired my whole life (still do frankly). But he hasn't wanted to be alive for a long time now (since his wife died ~15 years ago, and severely since he's been wheelchair bound). And he just keeps living. There's no way out for him.
I don't think we're designed to live that long. I definitely understand wanting to get out while you still have control over that decision. I doubt anyone would begrudge it. You can even control to some extent the final experiences with family. It's beautiful in a way.