Kudos to New York Magazine's Dan Amira, who tracked down one of President John Tyler's grandsons (not great-great-great-grandsons) to discuss the fact that he's still living. Here's a brief excerpt from Amira's interview with Harrison Ruffin Tyler:
It’s a really interesting story that you’re still, you know, around. Could you just explain how this happened? How someone born in 1790 still has living grandchildren? Well, he was a good man! [laughs] Both my grandfather — the president — and my father, were married twice. And they had children by their first wives. And their first wives died, and they married again and had more children. And my father was 75 when I was born, his father was 63 when he was born. John Tyler had fifteen children — eight by his first wife, seven by his second wife — so it does get very confusing. I really do not know — it’s amazing how families drift apart. When I was a child, I did know most of the descendents, but as you get more generations down the line, it’s hard to keep track of everybody.