I guess it all depends whether your descent is largely from the "left hand side" or the "right hand side" of the tree - and how large the families are.
I am largely "left hand side" for quite a few generations. I was born 1967 / parents 1941 & 1943 / grandparents 1908, 1910, 1911 & 1915 / great grandparents 1883 x 2, 1884 x 2, 1890 x 2, 1891 x 2. So for a largely "left-hand end" genealogist born in the early 50s and therefore recently retired (or about to retire), 1877 would be a more natural birth date for a great grandparent than a grandparent.
BUT ... my one surviving great aunt is the youngest of 11 (whereas my grandfather, her older brother, was the second). There is 25 years separating their births! She is only 6 years older than my father but she is a generation back up the tree. She didn't have any children, but had she done so some of them could have been no older than me (or even quite significantly younger) and their grandparents would be my great grandparents.
Replicate this for two or three generations of "right-hand descent" and it doesn't take very long to get you there. Suppose somebody born in 1960 whose father was 50 when he was born ... father born 1910. Suppose HIS father was 45 when he was born ... grandfather born 1875. These are not extraordinary ages at all.
I am, however, intrigued by the "left had side" / "right hand side" phenomenon ... it is really marked just how many "left hand ends" there are in my tree when you would have thought that there really ought to be a more or less even distribution.
Has anybody else found this?
Has anybody got even a working hypothesis for an explanation?