A popular bit of trivia is that US President John Tyler, born in 1790, has two living grandsons, born in 1924 and 1928. How many men born before 1790 have living grandchildren, and who is the oldest?
Based on the fact that the world’s oldest fathers are around 90, and the oldest people still alive are in their 110s, anything back to the 1720s is technically possible, however unlikely.
Not an answer to your question, but this newspaper article claimed the last grandchild of a signer of the Declaration of Independence died in 1921:
Aren’t there still children of Civil War veterans around today?
Two of those children are collecting civil war pensions from the government:
I think a few widows of veterans survived into the 2000s.
I’m guessing that your question is asking, “What is the furthest back date of birth for a grandparent of a living person?” In any case, I don’t have an answer.
I don’t have an answer, but I always like to pop into these threads and name-drop that one of President Tyler’s grandsons is a friend of mine. He goes to our church. He’s a fascinating gentleman.
According the New York Times, about 0.1 percent of children are born to fathers over 60, which is more than 2 000 a year these days in the US. There about 70 000 centenarians in the US. And children of older fathers apparently tend to live longer. So if the rate of births to older fathers hasn’t changed that much, you would think there’s probably a few people with a grandparent older than President Tyler somewhere in the US.