I think the ticketing systems are probably the most modern parts of airlines. As far as I know, the tech that actually runs the plane does not change very often as it needs to go through approval processes.
The ticketing system might very well be the oldest.
AFAIK the very first large-scale commercial deployment of what we now call "distributed cloud apps" was SABRE, a ticket reservation system built back in 1960s, still in use today.
