Short answers, pulled from the story.
The Pax Romana lasted from 27 BC to 180 AD, a span of roughly two hundred years. It is traditionally dated from Augustus becoming Roman emperor after the Battle of Actium in 31 BC, with the standard starting point set at 27 BC to exclude the preceding civil wars. It ended with the death of Marcus Aurelius in 180 AD.
The Roman Empire reached its greatest territorial extent under the emperor Trajan in 117 AD. At its population peak during the Pax Romana, the empire held an estimated 70 million people, which represented approximately 33 percent of the world's total population at the time.
The Five Good Emperors were the last five rulers of the Pax Romana, ending with Marcus Aurelius, whose death in 180 AD marked the close of the era. The source identifies them as a group by that name without listing each individual emperor.
The Pax Romana ended with the death of Marcus Aurelius in 180 AD. The historian Cassius Dio described what followed as a descent from a kingdom of gold to one of iron and rust, referring to the dictatorial reign of Commodus, the Year of the Five Emperors, and the Crisis of the Third Century.
The first known written use of the term Pax Romana appears in a text by Seneca the Younger, composed in AD 55. Arnaldo Momigliano later described the phrase as a simple formula for propaganda but a difficult subject for research.
Augustus defeated Mark Antony and Cleopatra at the Battle of Actium on the 2nd of September 31 BC and became Rome's first citizen, or princeps. He built a coalition of leading military figures to prevent civil war, used skillful propaganda to promote the benefits of peace, and ceremonially closed the Gates of Janus in 29 BC and again in 25 BC, with a third closure dated to 13 BC marked by the commissioning of the Ara Pacis.