1 Peter 1:23-25, KJV
Why not sin?
You have been born. Again. This time not of Adam, characterized by a flesh destined to perish because of sin, but of the Word of God. This was planted in our hearts when we first heard the gospel. The "flower of youth" lasts more briefly than your whole life, just as wildflowers bloom for a shorter lifespan than the stems beneath, which soon wither. Corruptible seeds are good for only so long, but the incorruptible Word of God bears eternal fruit. Your eternal life with God is its fruit! You don't have to walk around characterized by the corrupted flesh, because you are the one who was born brand new!
Verse 24 is quoted from Isaiah 40:6-8. Isaiah 40:26-31 continues from that starting point: don't bemoan your limitations due to your weak flesh. Let God raise you up to run and not be weary, and walk without becoming faint.