Hebrews 5:5 So also Christ glorified not himself to be made an high priest; but he that said unto him, Thou art my Son, to day have I begotten thee.
6 As he saith also in another place, Thou art a priest for ever after the order of Melchisedec.
7 Who in the days of his flesh, when he had offered up prayers and supplications with strong crying and tears unto him that was able to save him from death, and was heard in that he feared;
8 Though he were a Son, yet learned he obedience by the things which he suffered;
9 And being made perfect, he became the author of eternal salvation unto all them that obey him;
That is kind of surprising. I had no idea that "in the days of his flesh" Christ offered up prayers and supplications with strong crying and tears unto him that was able to save him from death, and was heard in that he feared. That is interesting also because Joseph Smith said that as a youth Christ was subject to death.
"When still a boy [Jesus Christ] had all the intelligence necessary to enable Him to rule and govern the kingdom of the Jews, and could reason with the wisest and most profound doctors of law and divinity, and make their theories and practice to appear like folly compared with the wisdom He possessed; but He was a boy only, and lacked physical strength even to defend His own person; and was subject to cold, to hunger and to death. (Teachings of the Prophet Joseph Smith page 392)
So Hebrews 5 and Joseph Smith both agree that death was something Christ was initially subject to. Joseph Smith says that he was also subject to cold and hunger, and Hebrews 5 says "though he were a son, yet learned he obedience by the things which he suffered",
It also seems that Hebrews 5:9 is saying that Christ's atonement required him to progress from grace to grace as described in D&C 93. That is, he had to be made perfect to become the author of eternal salvation. He could not have performed it as a boy. He had to progress from grace to grace until he had received the fullness of the father and power over death, and he states in the new testament that power over death was something he received from his Father. That fits with the rest of the Hebrews 5 description. It was during his mortality that the Father said to him "Thou art my son, to day have I begotten thee." It was during his mortality that the Father said to him "Thou art a priest for ever after the order of Melchisedec." He progressed from grace to grace in the usual way, both laying out and following the same path as everybody else. And at some point he received a commandment of his Father that he could lay down his life and take it up again.
John 10: 17 Therefore doth my Father love me, because I lay down my life, that I might take it again.
18 No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again. This commandment have I received of my Father.