Many modern animated TV series use a floating timeline, in which the characters do not age, but the time period in which the show is set adjusts to match when the episodes are created. The Simpsons is a famous example.
What's the oldest example of this device in literature in general? The Wikipedia article mentions the first Archie comic, which goes back to 1942.
It seems like an interesting question to me, because it requires a medium (like TV) that continues long enough for the device to be relevant, and also it requires having a noticeably changing time period to begin with.