There could be use for a way to know if the city is player's oldest one, in the requirement system. E.g. some ruleset could make that one the capital (/in lack of Palace)
The problem is how to determine which ONE is the oldest in case of multiple ones founded at the same turn. I guess looking at the city id would work, but is very ugly. And if we would want LongestOwned instead (so that ancient city captured just this turn would not turn to a Capital), the id won't help.
Maybe should aim for a more generic feature that uses city counters. Like requirement: "GreatestCounterValue", "Owned", "Player"