In the episode where Flanders announces that he is sixty. Okay, so he's sixty. But, in the episode where Flanders goes insane, this episode shows his life as a child in the 60's. If he's sixty why did he grow up in the 60's?
Flanders parents are clearly beatnicks rather than hippies, dating them to the fifties rather than the sixties. also the child flanders could be around five years old at the time of the flashback, placing his actual birth around 1945, very close to the correct time.
It's best not to focus on how they manage time in the Simpson’s... it's not consistent. Like the kids being in the same grade and with the same teacher even though they showed numerous episode where the year has changed, etc, etc… There are plenty of examples like these. But I would consider your slip-up a “slip-up”.
The Simpsons doesn't stay consistent between episodes! The episode where he is 60 is simply pointing out that he looks young because he never sinned. Where he is growing up in the 60's is a DIFFERENT EPISODE! Why can't people realise that?