As someone that is about 50, we also had it this way in Europe.
Newer generations don't get how lucky they are to have been born into EU, appreciate it while it lasts.
I'm almost 50 and from Europe, never had to think about this stuff for a second.
As a 29 year old that experienced EU citizenship then had it cruelly taken away by some stupidly thin margin of voters… feckin Brexit.
I get how lucky I was for 25% of my life expectancy.
Too bad there aren't too many people in their 90s around to discuss how things operated prior to WWII. Schengen was a solution to a problem that had only been created a few decades earlier.
Schengen is NOT a EU achievement.
Nations can sign Schengen, but are never forced to join the EU, nations can be EU members but are allowed to refuse the Schengen treaties.