Lord Olleus said:
Actualy i think that ages determine how much damage siege weapons do. I've noticed that when my cannons bombard a medieval age city they take away more of the city's defences then when they bombard an industrial age city. Might be wrong though.
Siege weapons take away a certain fraction of the city's defense. That means the higher the base defense, the more percentage points the siege weapons will take away, and the later you are in the game, the higher city defenses are liable to be. Thus, the later the age, the more percentage points your Cannons are likely to be removing.
Example:
Cannons will take away 20% of a city's base defense. Thus:
1 Cannon firing against 20% base defense: -4% per shot
1 Cannon firing against 50% base defense: -10% per shot
1 Cannon firing against 100% base defense: -20% per shot
etc.
Edit: SPQR300
Some Wonders are limited by what Age you
start in. Like Sirian said, the Ages serve as "entry points", meaning you can start a game that begins with everybody already in that Age, potentially disallowing those restricted Wonders. For example, you can choose to start with everybody already in the Industrial Age, but Stonehenge is only available on "Medieval and earlier" starts, so nobody could build Stonehenge in that game.
AFAIK, you also can't build a Wonder that is obsolete, i.e. Stonehenge after you get Calendar. However, this would only be tied to the Age indirectly via where the obsoleting tech is in the tech tree, with the Age itself
not being the determining factor.