For all age groups.
Who is in need of conversion? We usually think of people who have behaved in an extremely bad way or who don't know God at all. The passage quoted above is a famous proof text in order to encourage non-Christians to let Jesus into their lives.
We find these words in the last book of the Bible - the Revelation of John. In chapters 2 and 3, the Lord Jesus has a special message to certain churches in Asia Minor. It was a critical situation for Christians toward the end of the first century: There was persecution from the outside, at the same time false teachings threatened the church from the inside. It was a message for people who already were Christians.
One of those churches met in Laodicea - in the southwest of what is Turkey today. The content of the letter to this church (chapter 3, verses 14-22) leaves no doubt about the fact that the Christians there had become unfaithful to God and were in a lost condition. Jesus calls them "lukewarm" (verse 16), "miserable", "poor", "blind and naked" (verse 17). He was no longer in their midst like he had promised to his true church in the beginning. (Matthew 18:20) Instead he stood outside at the door (verse 20). His knocking and asking to be let in is symbolic for the command to repent which we can literally read about in verse 19.
It is not about non-Christians who simply open the "door to their heart" and experience salvation in that same moment. Rather it is about Jesus who stands at the door of an apostate church, asking the Christians there to turn around.
For the non-Christian there are some additonal requirements according to God's word: If Christ is to live in one's heart, there must be faith (Ephesians 3:17). This faith is produced through an honest encounter with the Bible message (Romans 10:17). It is not a theoretical faith but results in obedience (John 3:36; Hebrews 5:9). This includes of course turning awayx from old destructive habits and from egotism. This step is so radical that it is compared with Jesus' dying on the cross (Galatians 5:24). Salvation would be impossible without the death of Jesus and at the point of conversion we fully trust in the grace of God's saving hand (Ephesians 2:8). Unfortunately, this marvellous picture has been distorted by unbiblical teaching. People are told to pray a "sinner's prayer" although God's word shows a different way: the burial of the old man with Christ in baptism (Romans 6:3-7). All who begin their new live with the risen son of God in this way, can rely on the promise made by Jesus in John 14:23: