Freeciv

Cities and units have an "Age" and that's great for a lot of things. But not all things. Here's an example:

It might be really annoying that you can't conquer a remote city with some old cavalry units, because someone is going to rush it and steal Superconductors. Never mind the cavalry aren't even carrying superconductors on their horses ;)

Having a req for the TurnsOwned of a city could prevent that action from taking place. But that is only one example. It could just as easily be used in other ways like, city can't rapture until you've owned it for a while, or has 1 extra unhappy until you've owned it 5 turns, or can't produce military units for x turns, or has 50% higher corruption on the first turn owned, etc. etc.

Forgive if the examples are bad, I was just trying to show this could be a powerful feature not just for small complications, but for tuning the balance and and dynamics for the whole 'feel and flow' of how the game play comes together.