The synoptics use to group many teachings of the Lord in the same block. To know whom Jesus was talking to, you have to read verses before and after Luke 21:16-19. Nothing says he was talking to the twelve only. The Catechism of the Catholic Church also sees this passage as refering to the persecussion of christians at all ages :
- Before Christ's second coming the Church must pass through a final trial that will shake the faith of many believers (Cf. Lk 18:8; Mt 24:12). The persecution that accompanies her pilgrimage on earth (Cf. Lk 21:12; Jn 15:19-20) will unveil the "mystery of iniquity" in the form of a religious deception offering men an apparent solution to their problems at the price of apostasy from the truth. The supreme religious deception is that of the Antichrist, a pseudo-messianism by which man glorifies himself in place of God and of his Messiah come in the flesh (Cf. 2 Thess 2:4-12; 1 Thess 5:2-3; 2 Jn 7; 1 Jn 2:18,22)
If you want Jesus talking about the death of the Apostles (namely Peter and John), you should read John 21:18-24
Amen, amen I say to thee [Peter], when thou wast younger, thou didst gird thyself, and didst walk where thou wouldst. But when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and lead thee whither thou wouldst not. And this he said, signifying by what death he should glorify God. And when he had said this, he saith to him: Follow me.
Peter turning about, saw that disciple whom Jesus loved following, who also leaned on his breast at supper, and said: Lord, who is he that shall betray thee? Him therefore when Peter had seen, he saith to Jesus: Lord, and what shall this man do? Jesus saith to him: So I will have him to remain till I come, what is it to thee? follow thou me. This saying therefore went abroad among the brethren, that that disciple should not die. And Jesus did not say to him: He should not die; but, So I will have him to remain till I come, what is it to thee? This is that disciple who giveth testimony of these things, and hath written these things; and we know that his testimony is true. [D-R]