Originally Posted by
212man
They didn’t. PF and PM describe the roles of the crew members on the particular sector. Capt and co-pilot describes their status in the cockpit.
However, the term 'co-pilot' has generally been replaced by the term 'First Officer', just as the questioner, 'sci-fi' correctly alluded to. They were simply asking when that happened. I didn't see that they needed to be corrected, as they don't appear to be confused about captaincy / FO distinction vs PF / PNF roles on each flight.
To try to answer sci-fi, I believe it was during the 90s and since, however the timing varies considerably by country / culture, I believe, and I am happy to be corrected.