In Age of Empires III, there are five ages in which you can improve your civilization: the Discovery Age, Colonial Age, Fortress Age, Industrial Age and Imperial Age.
The Discovery Age is a generalized term to refer to the era during the 16th century in which European civilizations had attempted to circumvent the Ottoman Empire in favor of another economic route to India and Asia. In doing so, the European civilizations which took part, foremost consisting of Spain, Portugal, England and France, ‘discovered’ other inhabited continents relatively unknown to European influence.
In the Discovery Age, players start with a Town Center, several settlers and an Explorer. The Explorer, who replaces the scout from previous games in the Age of Empires series, is an unkillable unit which has the special ability of sniping environmental hazards. The Discovery Age is one of the most important eras for discovery because you are incapable of building any military. It’s relatively peaceful, but since it’s so easy to get to the Colonial Age, the peace ends quickly. Exploring and food-gathering are the most common tasks.
Cost: 800 Food
The Colonial Age is used to describe the period directly after the Discovery Age, in which the European civilizations that took part in the discovery process, including civilizations which hadn’t, began to populate and build settlements in the newly discovered continents. Colonialism consisted of economic and political domination over a regions resources, labor and regional markets by an already established state.
In AoE III, the Colonial Age is a key period in which players may actively interact with their Home Cities and local Native American tribes. They may also attempt to attack or limit the growth of an enemy civilization, hoping to take their fragile colony out early.
Cost: 1200 Food, 1000 Gold
The Fortress Age is most likely referring to a period in the mid-1700’s in which European nations desired to strengthen their positions in the North American continent by creating large forts and by using heavy artillery.
The Fortress Age in AoE III allows for the continuation of developments made in the Colonial and Discovery Ages. By the time a player reaches the Fortress Age, military improvements and economic improvements exponentially increase. Heavy artillery is available to the players, and that alone makes this age deadly. Large-scale battles are not uncommon.
Cost: 2000 Food, 1200 Gold
The Industrial Age may refer to the Industrial Revolution which took place during the 18th and 19th centuries. The Industrial Revolution was a massive economic, social and technological change facilitated by the introduction of steam power and the use of automated machinery in manufacturing. Although the Industrial Revolution started in Europe, the effects eventually spread worldwide with the advancement of transportational, economical and communicational technology.
In AoE III, the Industrial Age sees the growth and peak of militaristic and economic developments. By this period in the game, most players tend to have many outposts and are easily capable of producing large armies of militaristic units. Their civilization is usually very well developed to have been able to survive the past three ages, and most skirmishes end by this age.
Cost: 4000 Food, 4000 Gold
The Imperial Age is most likely refering to the acquisition of an empire, historically in regards to the British Empire of the 19th century. The Imperial Age focused on a policy of expanding a nation state across the world in the desire to gain economic and political superiority through the use of mercantilism, war and diplomacy.
The Imperial Age in AoE III is a crucial era in determining who wins and who loses a game. Considered the breaker of a long, stalemated game, the Imperial Age is expensive, though highly desirable. The technological improvements available to a player are massive, and can make them virtually unstoppable.