Tangentially, I started my family a bit late in life, as such my daughter will be barely into her 30s by the time I reach my 70s. I started a journal for this reason - so maybe whatever family she has can know me. Might be a tad narcissistic, but I regret not having know my own grandparents very well, so maybe they will appreciate one day.
Thats not narcissistic but a very good idea. My wife is an orphan, and especially now we have our own kid, we wonder a lot about how her parents would think about this or that.
I never even got to meet one of my grandparents. All I have is stories, and honestly not a lot of those.
I don't think you should feel narcissistic at all. Family is one of the prime places where we share our wisdom and values. Where we pass on the results of our mistakes in the hopes that the next generation avoids them. Don't feel bad for trying to do that, it's one of the fundamental elements of human progress.
I have the same age difference with my father. I have always been a melancholic and curious person so I ask a lot of questions. I think most people will have those questions that naturally come to them at some point. When you realize your parents are people with their own memories, hopes and flaws, a step that's probably called adulthood.
A very common trigger for this, as I've asked about this topic to other people with elderly fathers, is becoming a parent. But for me it was leaving home.
Also make room for such conversations. We had a month long vacation, just the two of us, which I remember fondly, even though there was no big event or deep discussion.
I was born late in both my parent's lives, and even though I'm only in my 40's, they are both now gone. The letters my folks wrote home while we were stationed overseas are one of my personal treasures. You might be writing for grandchildren, but don't downplay how important these artifacts are, even for children who already have spent years getting to know you!
Reading the prosaic thoughts of my parents when talking to other adults was like peeking behind the curtain, and it helped me understand them better as people, not just Mom and Dad. I am grateful for it.
