Classic Rendezvous uses 1983 as the cutoff year because in 1984 everything changed: Campagnolo died and junior immediately started taking the company in a different direction, Shimano introduced SIS and indexed shifting took off, Look invented the strapless pedal and toe clips and straps (supposedly) immediately became obsolete - those were the majors, and there were a few other changes, but '83 was definitely an end of an era that stretched back to at least 1945.

As to antique, I tend to go with the 25 year cutoff although as a rough line rather than something inflexible. That's always worked in antique cars and motorcycle for me, so it should work here.