1. There is an obvious contrast between the present age and the age to come in verse 18 (cf. espec. 2 Cor. 4:16-18).
2. Since ktisis in Greek can mean either creation or creature (cf. Rom. 1:25; 2 Cor. 5:17, etc.), it is important to let the sense determine the meaning of verse 19. Thus, since the latter derives from the former, contrary to the KJV it is clearly inclusive and so teaches that the entire material creation (which includes the fleshly creature) has of set purpose been subjected to corruption (decay) by God himself. It is an observable as well as a biblical fact that everything in this world ages (Mt. 6:19f.; Heb. 1:10f.). Obsolescence and death are universal (2 Cor. 4:16-18; Heb. 8:13) and need to be escaped from or transcended (cf. 2 Tim. 1:10). In contrast with its Creator who is both immortal and incorruptible (1 Tim. 1:17; 6:16), this world has both a beginning and an end (see e.g. Gen. 1:1; Isa.40:6-8; 51:6; 54:10; Mt. 28:20; Heb. 12:27; 2 Pet. 3:10). As creation in miniature the creature, including especially the sinless Jesus who was flesh (Luke 2:42; 3:23, etc.), grows older as it follows creation’s pattern.
3. The ‘also’ (Gk ‘kai’, which is inexcusably ignored in some modern translations where exegesis is governed by hidden assumptions and prior commitment) in verse 21 indicates that there is a change of subject. This assertion is supported by the ‘also’ in verse 23 following the indisputable reference to the whole creation in verse 22. Here the ‘also’ points up a change from ‘the whole creation’ (v.22) to ‘we ourselves’ (v.23) or, in other words, from ‘creation’ to ‘creature’. The only reasonable inference we can draw from this is that verses 21 and 23 correspond or provide a parallel. Otherwise expressed, these verses (cf. e.g. 1 Corinthians 15:50), supply yet another example of synonymous parallelism where verse 23 explains verse 21.
My conclusion is then that just as verses 19,20 and 22 refer to the whole creation, so verses 21 and 23 refer to the creature.
4. The reference to birth pangs in verse 22 reminds us of Jesus’ words in Mark 13:8. It is noticeable that in Mark 13:1-31 both creation and creature play their part. While the material heaven and earth pass away, Jesus’ words promising salvation to spiritual man do not (13:27,31).
5. While we can appreciate Paul’s claim that the whole creation can long eagerly for or anticipate the revealing of the sons of God (v.19), it is more than difficult to see how it can itself ‘obtain’ the freedom of the glory of the children of God (v.21). In light of 8:14-17, it is the creature man who will do this (Rom. 8:30) and, having shed his corruptible flesh (1 Cor. 15:50), thereby gain his eternal inheritance (cf. Acts 13:34; Heb. 9:15).
6. The subjection in verse 20 is purposeful, for it is said to be ‘in hope’ thereby precluding any reference to sin as a cause. Since this hope, according to verses 24 and 25, is invisible, we are forced to conclude that it refers to the creature man (cf. Heb. 11:1; 1 Pet. 1:8f.) who is made in the image of God and not to creation in general. This inference is necessary since according to the NT (a) the visible, that is, the material, is incapable of being eternalized (1 Cor. 15:50b); (b) it is temporary by nature (2 Cor. 4:16-18, cf. Rom. 2:28f.); and (c) all created things will eventually be destroyed (Heb. 12:27; 2 Pet. 3:5,7,10-12). The present age will give way to the age to come, earth to heaven, the material to the spiritual.
The widespread idea that Paul teaches here the restoration or redemption of creation from the corruption that stemmed from Adam’s sin is false. If it is objected that the apostle refers unquestionably to the redemption of the body in verse 23, it can readily be replied that (a) our fleshly body was forfeit on account of sin (Rom. 8:10, cf. 2 Cor. 5:1); (b) that even the sinless Jesus as flesh had to be changed to enter heaven (1 Cor. 15:50-53), and (c) that we shall be given a spiritual or glorified body (1 Cor. 15:44,46; Phil. 3:21, cf. 2 Cor. 5:1) in accordance with the plan of God (2 Cor. 5:5).
See further my Romans 8:18-25, Not Only But Also, The Transience of Creation, Creation Corruptible By Nature, Death and Corruption, Two ‘Natural’ Necessities, Why the Biblical Stress on Invisibility?, John Stott on the Putative Resurrection Transformation of Jesus, The Correspondence Between Romans 8:12-25 and 2 Corinthians 4:7-5:10, Another Shot at Romans 8:18-25, Further Reflection on Romans 8:18-25 – An Alternative Approach
What is translated in verse 20 as ‘not willingly’ (ESV), ‘not of its own will’ (NRSV) or ‘not by its own choice’ (NIV) in light of Philemon 14 and 1 Peter 5:2 almost certainly means something like ‘of set purpose’ or ‘by divine decree’.