This is something so obvious yet in a deep way shocking to me.
Both my grandparents were born in the 1800’s. They were married before WWI. It was natural when I was growing up for older people to understand and know about the turn of the century (1800-1900). I remember one woman telling me about all the horses on the roads in NYC, and the coal furnaces they all used.
When I watched the Memorial Day parade it was full of WWII vets and they were fairly young men. There was always a contingent of WWI vets, older, but a group of about 8. My mom told me she used to see the Civil War Vets in the parades - they were old, but they were still around.
As Orwell says, the Past is a Different Country. All that knowledge disappears, and people just forget it, and everything changes.
My son, now 22, enjoys talking about his ancestors and the changes each of them saw in their lives. It helps personalize history and talking about time spans in terms of personal lifetimes makes it real.
My oldest daughter, now 26, studied in Sweden in college. She took a side trip to Gdansk in Poland and visited the childhood home of her great grandfather. Her great great grandfather was an entrepreneur who started a battery company there in the 1880s. Gdansk used to be Danzig, Germany and is only a few miles from where WW II started. That really got her interested in our family history.