I think the whole 'I don't know what it will be like when I'm a sprightly 40 and he's a 60 year old codger ' thing is a bit of a red herring.
It's easy to think ahead in terms of numbers - forgetting that there is a person attached to those numbers, and years of shared history.
I'm nearly 40, and my beloved dp is 55. He's funny, cool, smart and sexy - even at the grand old age of 55!
To be frank, a boring old fart will be a boring old fart at 21, 41 and 61 my stbxh for example
Whereas a man who is interested in you, life and the world around him is likely to remain so.
As far as ending up as a carer goes - well, that could happen at any age. There were days between my grandparents, and poor grandad ended up looking after my grandma and their two dds for years after she developed a serious heart condition - eventually losing her when they were both 36
There are no guarantees. Carpe diem!