This thousand-year reign of Christ, also called the Millennium, offers people who have not known Christ or His true expectations an opportunity to respond to Him. It is important to note that this is not a second chance. It will be the first and only chance for these people. The Bible teaches that everyone will have an opportunity to come to God (2 Peter 3:9 2 Peter 3:9The Lord is not slack concerning his promise, as some men count slackness; but is long-suffering to us-ward, not willing that any should perish, but that all should come to repentance.
American King James Version×).
For those called today, the firstfruits, the opportunity (judgment) is now (1 Peter 4:17 1 Peter 4:17For the time is come that judgment must begin at the house of God: and if it first begin at us, what shall the end be of them that obey not the gospel of God?
American King James Version×). Others will have this opportunity during the Millennium.
As for those who are alive at the start of the Millennium, these people are ones who will have lived through the cataclysmic events of the return of Jesus Christ—called the "day of the Lord" in the Bible. Their physical lives will continue as the Kingdom of God is established here on earth (Revelation 5:10 Revelation 5:10And have made us to our God kings and priests: and we shall reign on the earth.
American King James Version×).
As to what the life span of human beings during the Millennium will be, the Bible doesn't explicitly say. It probably will be at least as long as people live today, and perhaps longer (see Zechariah 8:4 Zechariah 8:4Thus said the LORD of hosts; There shall yet old men and old women dwell in the streets of Jerusalem, and every man with his staff in his hand for very age.
American King James Version×). People will have children (verse 5). One important change in human life during this time, however, will be the interaction of the spirit world with human beings.
During the Millennium, Satan will be bound (Revelation 20:1-2 Revelation 20:1-2  And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.
 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
American King James Version×) and the firstfruits, God's people who will have been changed into spirit at Christ's return (1 Corinthians 15:52 1 Corinthians 15:52In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
American King James Version×), will be actively teaching people God's ways.
Commenting on this time, Isaiah 30:20-21 Isaiah 30:20-21  And though the Lord give you the bread of adversity, and the water of affliction, yet shall not your teachers be removed into a corner any more, but your eyes shall see your teachers:
 And your ears shall hear a word behind you, saying, This is the way, walk you in it, when you turn to the right hand, and when you turn to the left.
American King James Version×says: "And though the Lord gives you the bread of adversity and the water of affliction, yet your teachers will not be moved into a corner anymore, but your eyes shall see your teachers. Your ears shall hear a word behind you, saying, 'This is the way, walk in it,' whenever you turn to the right hand or whenever you turn to the left."
Satan's constant influence to cause people to sin, which is part of this present evil world, will be replaced by spirit beings who will help guide people into godly paths.
Because of this positive teaching and guidance by spirit beings and the fact that God's law will be going out to all the earth from Jerusalem (Isaiah 2:2-3 Isaiah 2:2-3  And it shall come to pass in the last days, that the mountain of the LORD's house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow to it.
 And many people shall go and say, Come you, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem.
American King James Version×), it seems that people living in the Millennium will know about this period of time and that this is their opportunity to respond to God.