If a man were called to fix the period in the history of the world, during which the condition of the human race was most happy and prosperous, he would, without hesitation, name that which elapsed from the death of Domitian [96 CE] to the accession of Commodus [180 CE]. The vast extent of the Roman empire was governed by absolute power, under the guidance of virtue and wisdom. The armies were restrained by the firm but gentle hand of four successive emperors, whose characters and authority commanded involuntary respect.
Edward Gibbon, The Decline and Fall of the Roman Empire, Chapter 3
Gibbon doesn’t include China in this assessment of the state of the world, but for that country too, under the Eastern Han dynasty, there was a period of stability and prosperity, lasting from the death of the usurper Wang Mang in 24 CE to the outbreak of the Yellow Turban peasant uprising in 184 CE. During this time, the Roman and Han empires so completely dominated their respective portions of Eurasia that they enjoyed relative peace. Toward the end of the second century CE, both empires had populations around 50-60 million; world population was perhaps 190 million. In the succeeding centuries both empires would experience major population declines and political collapse. As a result, the world’s total population may have declined as well.
Of course Gibbon’s view is a retrospective one, and didn’t anticipate the vast rise in standards of living that eventually followed the industrial revolution.
(After this I’ll give dates as numbers without the “CE”.)