Temple/Library Culture Bug

[Fistleaf]

Has anyone noticed that the culture production for a temple doubled from 2 to 4 after some other improvements were built (I suspect it's the Colosseum)?
Also, the culture production for library doubled from 3 to 6 for some cities when the cities were similar in size and improvements.

[Bill3000]

It's not a bug, it's a feature.

It's the age bonus of improvements.

[Fistleaf]

So how does it work?
Is it mentioned in the manual?

[Laszlo]

Yeah, the older a thing is, the more culture it generates. On page 113-114 of the Civ3 Instruction Manual it says, "The longer a thing exists, the more venerable it becomes, and thus it contributes more," though it doesn't go into specifics on the amount or when, though it does add that culture generation receives a 50% cut during war.