Luke 9:27 and several other verses (e.g. Matt. 10:23; 16:28; Mark 9:1; etc.) seem to suggest the same thing: some of the people who heard Jesus teach would still be alive when he brought the kingdom of God. Part of the problem in understanding these verses is that the Bible speaks in a number of different ways about the kingdom of God. In some senses, the kingdom of God is already here, but in other senses it is not. For example, in Matthew 12:28 // Luke 11:20 and Luke 17:21 Jesus indicates that the kingdom of God has already come. In Luke 16:16 Jesus indicated that people were already entering the kingdom of God. And in Revelation 1:6 John proclaimed that he and others were already part of the kingdom.

In fact, many theologians (myself included) believe that one of the main purposes of the New Testament is to explain the fact that the kingdom of God has come and is coming in a way that the Old Testament did not anticipate. Jesus inaugurated the kingdom during his earthly ministry, but he did not bring it in all its fullness. Rather, the coming of the kingdom of God is now a long, drawn-out process. It began to come when Jesus first came, it continues in its partial coming right now, and it will finally finish coming when Jesus returns. This is largely the gist of many of Jesus' "kingdom parables" (e.g. those in Matt. 13), such as the sower, the mustard seed, the wheat and the tares, etc. The kingdom must grow to fullness and maturity; it does not all come at once.

As another proof that the kingdom has come, Jesus is right now reigning in heaven (Matt. 28:18; 1 Cor. 15:22-27; Eph. 1:20-23; Col. 3:1; Heb. 1:3), as well as on earth through his church (Matt. 28:18-20; Eph. 1:20-23). He has already disarmed and triumphed over his enemies (Col. 2:15), though he has not finally destroyed them (1 Cor. 15:25-26).

Thus, it is absolutely true that many people who heard Jesus teach also saw the kingdom of God. They saw Jesus defeat his enemies at the Cross, rise from the dead in victory, and ascend to his throne in heaven. Though none of us has seen the final coming of the kingdom yet (we await that at Christ's return), it is still true that all of us who are saved have been rescued from the domain of darkness and transferred to the kingdom of God (Col. 1:13; cf. Matt. 19:24-26 // Mark 10:25-27 // Luke 18:25-27). And in a broader sense, it is also true that all who are in the church are in the kingdom of God, though the unbelievers in the church stand in line to be condemned by the king (1 Pet. 4:17).

In short, sometimes when the Bible refers to the "kingdom of God," it is referring to aspects of the kingdom that have already come, and other times it is referring to aspects of the kingdom which have not yet come. Certainly, Luke 9:27 came true in many ways, including the fact that many of the people who heard these words (e.g. eleven of the disciples) saw the resurrected Lord ascend to his heavenly throne.