The Age of Terra, also sometimes called the Age of Progress in older Imperial texts, is the era of human history between the 1st and 15th Millennia (1 - 14,999 AD) when Mankind advanced from a pre-industrial Iron Age culture to become a star-spanning Fusion Age civilisation.
The original reason for numbering the years from this point is because it was believed to be the year of the birth of the ancient religious leader Jesus Christ, which has long been forgotten by humanity, which now possesses only one religion -- a deep faith in the God-Emperor of Mankind and his Imperial Cult. During the long years of the Age of Terra, humanity came to dominate the Earth culturally and technologically. Myriad human civilisations came and went during this long span of now mostly forgotten human history.
The first three Chaos Gods -- Khorne, Tzeentch and Nurgle -- were birthed in the Immaterium during this period, all coming into existence at various points during Old Earth's European medieval period. Their emergence at this time was the result of the constant warfare, the growth of sophisticated early modern nation-states and continued outbreaks of global plagues, respectively, which marked that era and fed the final formation of the Dark Gods' consciousnesses.
The Solar System was ultimately colonised once humanity developed the necessary technology to leave the Earth's orbit in the late 2nd Millennium and the early 3rd Millennium. Mankind first colonised Mars and lived on the moons of Jupiter, Saturn and Neptune. Mars became the first planet to be terraformed by humans when it was settled by various industrial cartels. The Red Planet soon became synonymous with technical expertise and scientific advancement, a characteristic that would mark it for many millennia to come. Advanced fusion and anti-matter propulsion technology was developed that eventually allowed humanity to construct interstellar starships that could colonise other star systems, though the slow travel times of these vessels meant these human colonies had to be self-sufficient to survive, as no aid could arrive from Earth in time.
At the very end of the period in the 15th Millennium and stretching into the next era in human history, the Age of Technology, humanity began to colonise the stars using these sub-light spacecraft. At first only nearby star systems could be reached and the human colonies established on them were required to survive as independent states since they were separated from Earth by up to ten human generations' time of travel.
Almost nothing of this period's events is known to current Imperial historians save as myth and legend, though it is clear that the Emperor was alive and on Earth throughout this period. He is believed to have been born in the 8th Millennium B.C. in Anatolia, then disguised His true abilities and immortal nature as He moved from identity to identity throughout Old Earth's ancient history, always trying to better the lot of humanity, physically, intellectually and spiritually.
- Galaxy in Flames (Novel) by Ben Counter, pg. 280
- Realm of Chaos: The Lost and the Damned (1st Edition), pp. 174, 176
- Warhammer 40,000: Rulebook (8th Edition), pg. 40
- Warhammer 40,000: Rulebook (5th Edition), pg. 122