The Isaiah 65:20 Scripture is an expression for the world to come which begins on the "day of the Lord" when Jesus returns to gather His Church, i.e, the last day of this present world, and the start of the "thousand years" millennium reign by Christ over the nations per Rev.20.
These are all Millennium AND... later new heavens and a new earth Scriptures mixed together...
Isa 65:14-25
14 Behold, My servants shall sing for joy of heart, but ye shall cry for sorrow of heart, and shall howl for vexation of spirit.
15 And ye shall leave your name for a curse unto My chosen: for the Lord GOD shall slay thee, and call His servants by another name:
16 That he who blesseth himself in the earth shall bless himself in the God of truth; and he that sweareth in the earth shall swear by the God of truth; because the former troubles are forgotten, and because they are hid from Mine eyes.
17 For, behold, I create new heavens and a new earth: and the former shall not be remembered, nor come into mind.
18 But be ye glad and rejoice for ever in that which I create: for, behold, I create Jerusalem a rejoicing, and her people a joy.
19 And I will rejoice in Jerusalem, and joy in My people: and the voice of weeping shall be no more heard in her, nor the voice of crying.
20 There shall be no more thence an infant of days, nor an old man that hath not filled his days: for the child shall die an hundred years old; but the sinner being an hundred years old shall be accursed.
21 And they shall build houses, and inhabit them; and they shall plant vineyards, and eat the fruit of them.
22 They shall not build, and another inhabit; they shall not plant, and another eat: for as the days of a tree are the days of My people, and Mine elect shall long enjoy the work of their hands.
23 They shall not labour in vain, nor bring forth for trouble; for they are the seed of the blessed of the LORD, and their offspring with them.
24 And it shall come to pass, that before they call, I will answer; and while they are yet speaking, I will hear.
25 The wolf and the lamb shall feed together, and the lion shall eat straw like the bullock: and dust shall be the serpent's meat. They shall not hurt nor destroy in all My holy mountain, saith the LORD.
KJV
The Millennium is only about Christ's thousand years reign that begins at His return. But the new heavens and a new earth is for after the thousand years and begins after the GWT judgment.
The reason why those descriptions are mixed together there is because even beginning with Christ's thousand years, some eternal changes upon the earth are going to start to manifest then. A look at Rev.22:14-15 easily shows this, as God's River and the Tree of Life inside the city of Jerusalem on earth are manifest during Christ's thousand years reign, with the wicked dwelling outside the gates of the city.
On the day of our Lord Jesus' 2nd coming, all peoples alive on earth will be changed at the twinkling of an eye to the spiritual body. There will be no more... flesh death after that day. The only death remaining per Rev.20 will be the "second death", which is the perishing of one's spirit/soul into the lake of fire at the GWT judgment.