Rise of Nations/Ages

Ancient (ca 2000 BC)
The starting age of the game, the Ancient age offers only basic infantry and towers. Cities cannot increase above the Small Size, and neither Knowledge nor Metal exist yet. Instead, players will find most units and upgrades require Wealth and Timber instead. Any troops in this stage are weak, and shouldn't be used to attack enemies.

Classical (1999 BC - 500 AD)
Stables, Siege Factories, and Forts can be built, making it much easier to attack your opponents via mounted units and artillery. Cities can be increased to the Large size if there are at least five buildings in their area of influence. Knowledge and Metal now play a factor, and Universities and Mines can be built. Cavalry can now be trained and used to cut a path through the infantry. This is the age where the fighting becomes interesting. Also, buildings are now vulnerable with the introduction of Siege Weapons.

Medieval (501 AD - 1299 AD)
Forts become Castles, and Catapults can be upgraded to Trebuchets, which are significantly more powerful. Also, the key Supply Wagon unit is introduced (along with Spies), allowing for effective siege attacks. All unit types can receive upgrades. The Medieval age is considered the "height of the defense curve," as it gets easier to attack after the introduction of the Supply Wagon, and in subsequent ages.

Gunpowder (1300 AD - 1715 AD)
The introduction of Arquebuses, Bombards, and Dragoons pushes the balance of power toward the attacker. The introduction of Gunpowder is one of the most significant developments in history, and in the game. Teching up to Gunpowder may leave you vulnerable to early-age rushes, but those who survive to obtain gunpowder have a huge advantage over those who lack it.

Enlightenment (1716 AD - 1880 AD)
Heavy Infantry first gain ranged weapons in the Enlightenment Age. Every unit gains a significant upgrade, but the most notable is that of the Cannon. When upgraded, the Cannon's role can shift from being primarily anti-structure to a combination of anti-structure and anti-infantry.

Industrial (1880 AD - 1936 AD)
The period when steam powered machinery was introduced into the world. This gae paves way for the main offensive powers of the game. With the introduction of tanks and aircrafts this age paves way to a new vector of fighting. The main introduction is Armored Cavalry. Tanks and scout cars play a major role in cutting down the enemy infantry and cavalry.

Modern (1936 AD - 1968 AD)
Primitive unit types introduced in the Industrial Age now become potent weapons of war. More recognizable vehicles become available, including tanks, fighter aircraft, bombers, and cruise missiles. Since Tactical Nuclear Weapons make their first appearance in this era, it is a good idea to begin spreading your units out more to discourage your opponents using them on your forces.

Information (1969 AD - future)
All modern upgrades to Economy, Technology, and Military are now available, including Stealth Bombers, ICBMs, and powerful Main Battle Tanks. Once all technologies in the Information Age are researched, special additional technologies become available to bring the game to a dramatic finish.