This is a really interesting thread, thank you @Tuftily .
I am in my mid-50s, childfree by choice. It is only in the past few years that I have wavered (albeit much too late) on my choice, all throughout my childbearing years I never once felt broody, never looked at a baby and thought I might like to be a mum, I was always certain and thankful for my decision and hugely thankful that my lovely DH felt the same way.
I think it is completely understandable that as time marches on and the later years are looming to wonder if indeed you made the right choice and it is painful to think that perhaps you did not. I did go through a period of wondering this and largely, for me, it was as a result of losing my dad, knowing how much we loved him and cared for him in his illness, how it strangely brought us together, and knowing I won't have a child to feel that way about me or to advocate for me in the way that we did for dad.
However, I am somewhat out of this 'place' now and back to feeling more sure that I am on the right path for me. While I do sometimes look with a touch of envy on friends with families around them, I know that the life they had for 20 + years was not for me. Children are wonderful, it's not that I don't "like" them (what a strange notion, to "like" or "dislike" people based purely on age) but I never felt like it was the life for me. I love a quiet home, not much comings and goings. DH and I do lots together but also lots apart. I love exercising and walking and do some classes. There is a Womens Shed near where I live which I recently joined and even a Senior Centre (for over 55s!) which tons of activities going on - I haven't yet joined this but absolutely will in the coming years (if I am lucky enough to still be around). I enjoy company but I love coming home to my quiet house too.
I am lucky in that I have DH and a great sister (no nieces or nephews) and a few great friends, 2 of whom are also childless) but yes one day I may well be alone.
I think all you can do is to accept the choice you made back then was the right choice for you and that, while it comes with some downsides, the other choice would not have led to a 'perfect' life either - life ebbs and flows, sometimes it feels 'right', sometimes maybe not so much. Learning to accept 'what is' helped me throughout dad's illness, rather than wishing for 'what is not'.
I am, for the most part, back in a place of enjoying my life, enjoying moments and not giving too much thought to tomorrows.