Ages are the main hallmarks of technological development in Rise of Nations, and represent the periods of time in which human history has seen significant changes and/or development in the arts, sciences, technology, economy, warfare, religion and philosophy.
As you put on fresh new clothes and take off those you've worn, You'll replace your body with a fresh one, newly born. - Bhagavad-Gita (Cap II)
After enough research of the four main branches of technology has been carried out, an Age Advancement will be avaliable for research at the Library.
Advancing to a new age can be costly, but necessary if the nation is to remain competitive and be able to survive. For instance, age advancement allows for the introduction of newer, stronger military units, often impossible to defeat if the gap in techonology is wide enough. Age also increase the capabilities of a nation as a whole, allowing it to expand its influence further and dramatically increase its gathering rates and population. Most structures also unlock and upgrade along with age advancement, increasing the nation's economic, military and strategic potential. The vast majority of wonders can only be built if the technology and age requirements are met. An example is the Space Program, a wonder that can turn the tide of an entire game, can only be built in the Information Age.
Advancing through all ages also allows for future technology, the most powerful of technologies, to be researched.
If an opposing (not allied) nation has already researched an Age first, then you will receive a 20% discount on the Knowledge, and 10% discount on all the other basic resources on the cost for researching the next Age. If an opposing nation is two or more Ages ahead, then the discount doubles: 40% on Knowledge and 20% on all other basic resources to reach the next age.