Seriously? I know you're exaggerating to make a point, but ...
Current estimates are that there are at least 72000 people in the United States who are over 100 years old, with estimates that there will be close to a million by 2050. In the UK it's estimated that there are 9000 centenarians, with estimates of 40000 by 2031. The 2016 Canadians census counted 8230 centenarians. Japan has 30000, China's most recent census reported 7000. Based on demographic trends, both are projected to have numbers rivalling the of the US by 2050. All those people who are over a hundred years old obviously knew people who were alive a hundred years ago.
There are all sorts of people alive today who knew people who were alive a hundred years ago, and in a hundred years time there will still be people alive who are alive today.