I 55yo. I think any people 40s+ would have meet 90-100 yo family members in their youth. I lived in in a 1870s house in a town in Connecticut and most of the houses on Main St. were of that vintage. It is not so uncommon in New England. Of course percentage wise I imagine most people in the US (and else where) live in structures younger. The house I a in now (my father built) is about 50yo and my grandparents house (that we inherited) is 80yo+.

I spoke with an English engineer once who lived in a 400-500yo house in England and he knew an Egyptian who had lived in a house in the 1000s years old range. I suppose the terms "young" and "old" are fairly relative.