I have read this thread with fascination, as our family seem to consistently have our children relatively late. My grandmother was 27 when she had her first child, my mother was 31, I was 28 and DS was 35 (DDiL was 38).
I do not think coming late to grandparenthood has had any downsides. Although we were retired when DGC were born, as we live 200 miles away, rather negated that advantage and we are a healthy, long-living race. At 74, I was in the garden having a kickabout with 7 year old grandson this Easter so we can do all the things younger grandparents do and now DGC are both of school age, I find them far less tiring than smaller children.
As for great grand children...... well DF lived to be 92, healthy and well and living independently before succumbing to a short illness so lived long enough to see his great grand children. I hope to do the same myself.