"Some years ago I was in Washington, D.C., with President Harold B. Lee. Early one morning he called me to come into his hotel room. He was sitting in his robe reading Gospel Doctrine, by President Joseph F. Smith, and he said, “Listen to this!”

“Jesus had not finished his work when his body was slain, neither did he finish it after his resurrection from the dead; although he had accomplished the purpose for which he then came to the earth, he had not fulfilled all his work. And when will he? Not until he has redeemed and saved every son and daughter of our father Adam that have been or ever will be born upon this earth to the end of time, except the sons of perdition. That is his mission. We will not finish our work until we have saved ourselves, and then not until we shall have saved all depending upon us; for we are to become saviors upon Mount Zion, as well as Christ. We are called to this mission.”
“There is never a time,” the Prophet Joseph Smith taught, “when the spirit is too old to approach God. All are within the reach of pardoning mercy, who have not committed the unpardonable sin.”
Boyd K. Packer, “The Brilliant Morning of Forgiveness,” Ensign, Nov. 1995, 18

https://www.lds.org/general-conference/1995/10/the-brilliant-morning-of-forgiveness?lang=eng

Original Joseph F Smith quote here: https://www.lds.org/manual/teachings-joseph-f-smith/chapter-46?lang=eng