Age is an aspect of the server, where crafting is limited only to the age the server is currently in. For example the "Iron Age" where only Iron can be crafted as tools. Check for more information. The age was created to give values on blocks that are sometimes devalued in minecraft.
Animals don't magically spawn, there are only finite amounts of creatures in the world(except hostile mobs). These animals are possible to get extinct, for they do not respawn back.
Crop Growth are slowed in the game, this gives value to all those players who wishes to be one.