It's not just great grandparents, but the family calculus on grandparents changes significantly as well. If my parents were 35 when I was born, and I don't have children until 35, my parents are 70. With a life expectancy of 80, my children never really get a chance to know my parents. Whereas if each generation is having children at age 25, my children will likely be able to know their grandparents for 30 years.
I have no idea if it's good or bad, but it's interesting to think about. I do have to wonder if it affects how younger people perceive the past, since they have less of a direct connection to the past.
My mom's parents were relatively old when they had her, and my dad's parents were relatively young when they had him. I had a set of 'old' grandparents, and 'young' grandparents.
My mom's parents were gone by the time I was 24 - I didn't really ever get a chance to interact with them as an adult. My dad's father passed away when I was 39, but I had many visits with them as an adult while he and my grandmother were still pretty active. My grandmother is now 94 and not in great health, but still with us, still mentally there. When I was in 5th grade, they came to 'grandparents day' at my school, and she won 'youngest grandmother', but wouldn't come up on stage to accept the award! ;)
I realised last year that I'd reached the age (48) that my mums dad was when I was born. I remember him "always" being "old", but I also know that I have memories of him from before he turned 55 (he took early retirement around then, and I remember clearly the discussions about what would happen to his workplace that led up to the offer of taking early retirement). He lived another 32 years after that, and all of my grandparents were young enough through most of my childhood and teens that not only were they around but they had the energy to have us stay for whole weeks during the holidays and take us all kinds of places...
Meanwhile my son, at 14, has only one living grandparent, and it does feel weird.
Support networks are not necessarily limited to blood relatives.
(in particular, I was never within 5 hours of gp or ggp until my teens, and born multiple TZ away. then again, I'm in the "come home before it gets dark" generation; ymmv)
I have no idea if it's good or bad, but it's interesting to think about. I do have to wonder if it affects how younger people perceive the past, since they have less of a direct connection to the past.