It must have been longer than 20 years because:
1.) 9/11/01 was more than 20 years ago, which really locked down a lot of security protocols and introduced new ones.
2.) Even before 9/11/01, cockpit visits in flight/flying jumpsuit for non airline pilots was still strictly prohibited on US airlines. Maybe someone can chime in as to when US Airlines *may* have been doing this but I can't imagine it happening back to the 80's. I was fortunate to get a cockpit visit on an AF 747-200 in 1999 upon request. We were up over Greenland enroute from CDG-EWR and it was an amazing view. European airlines were known for allowing this at Captain's discretion. I've heard stories of passengers being given the amazing opportunity to visit the Concorde in flight. Super cool experience for sure.
I'm sure it was against the rules, but it certainly happened. I was a frequent flier some years as a kid, but for me Im sure it was more because I was a kid interested in aviation. These would have been domestic flights in the very end of the 80s, early 90s. I dont remember ever being denied a midflight visit to the cockpit. Do the math in your head for what the airspeed was in mach and they might let you sit in the right hand seat in flight. Or so it read in a book...
