Isa. 65-66 and Rev. 21-22. New Heaven and Earth

A few random thoughts:
Isaiah 25:6-8 speaks of the abolition of death 'on this mountain' i.e. the new Jerusalem; compare with 64:25; Revelation 21:10.
The NIV leaves out the initial 'for' in 64:17, which is unfortunate because the word connects 17-25 with what went before, to the promises of 8-10, the threats of 11-12 and the judgments of 13-16.
'New heavens and a new earth' represents the totality of things. (cf. Genesis 1:1). So the 'former things' not only picks up on the 'former troubles' of v.16, but also the old order as a whole. It's gone and rejoicing is the order of the day (vs.18-19).
So v.20, I think we have to interpret figuratively. If there were still death (contra 25:6-8), the voice of weeping would still be heard (v.19). So what it's saying is that whereas in this world, death cuts life short, in the NH&NE one would be a mere child if one died at a hundred, and a sinner 100 years old would be accursed, not that there are any sinners in the New Jerusalem (cf. vs.12,15) Isaiah is using metaphor to say that even if it were possible (it isn't) for a sinner to avoid detection for 100 years, judgement would still find him.
So verse 20 tells us that in the NH&NE, death will have no more power and sin no more presence.
[I've had quite a bit of help from The Prophecy of Isaiah by Alec Motyer here]

 

Isaiah describing the future Millennium under Christ, while John sees eternal state!