From Albert Barnes notes on the bible:
Verily I say unto you ... - To encourage them, he assured them that, though his kingdom was now obscure and despised - though he was cast out and little known - yet the time was near when he would be regarded in a different manner, and his kingdom be established with great power.
This cannot refer to the end of the world, and there is no need of referring it to the destruction of Jerusalem.
Taste of death - That is, die. Before they die they shall see this.
Son of man coming in his kingdom - Mark and Luke have explained this: Mar_9:1, "Until they have seen the kingdom of God come with power;" Luk_9:27, "Till they see the kingdom of God." The meaning evidently is, "till they shall see my kingdom," i. e., my church, now small, feeble, and despised, greatly enlarged, established, and spreading with great rapidity and extent. All this was accomplished. All these apostles, except Judas, lived to see the wonders of the day of Pentecost; some of them, John particularly, saw the Jewish nation scattered, the temple destroyed, the gospel established in Asia, Rome, Greece, and in a large part of the known world.