(The following is taken from DAVID GUZIK COMMENTARY ON THE BIBLE, Studylight.org)
(1 Peter 5:1) .
The elders who are among you I exhort, I who am a fellow elder and a witness of the sufferings of Christ, and also a partaker of the glory that will be revealed:
The elders who are among you I exhort: Peter will give a word of exhortation to the elders who are among the Christians reading this letter. These elders have special responsibilities that Peter will address.
The idea of the elder came into church life from Jewish culture (Exodus 3:16; Exodus 12:21; Exodus 19:7). The word “elder” simply speaks of the maturity and wisdom that an older person should have, making them qualified for leadership. In its application, it is more about wisdom and maturity than age.
It was the practice of Paul and Barnabas to appoint elders in the churches they had founded (Acts 14:23). There was also the development of the office of pastor, who was essentially a teaching elder (1 Timothy 5:17) who appointed and guided elders and other leaders (1 Timothy 3:1-13, 2 Timothy 2:2, Titus 1:5-9).
I who am a fellow elder: Peter is qualified to speak because he is a fellow elder. Though Peter was clearly the prominent disciple among the twelve, he claims no special privilege or position, such as the “Pope” of the early church. Instead, Peter sees himself only as one fellow elder among all the elders in the church.
A witness of the sufferings of Christ, and also a partaker of the glory that will be revealed: Peter is qualified to speak because he is a witness of Jesus’ sufferings, when he saw Jesus’ torture and crucifixion, and he was a partaker of Jesus’ glory, probably when he saw Jesus’ transfiguration.
Peter wrote about suffering and glory in the Christian life (1 Peter 4:12-13). Because Peter saw both suffering and glory in the life of Jesus, he can speak with the authority of an eyewitness.