Shakespeare's Sonnet 22

My glass shall not persuade me I am old,So long as youth and thou are of one date;But when in thee time’s furrows I behold,Then look I death my days should expiate.For all that beauty that doth cover thee,Is but the seemly raiment of my heart,Which in thy breast doth live, as thine in me:How can I then be elder than thou art?O! therefore love, be of thyself so waryAs I, not for myself, but for thee will;Bearing thy heart, which I will keep so charyAs tender nurse her babe from faring ill.Presume not on thy heart when mine is slain,Thou gav’st me thine not to give back again.

My mirror will not convince me I am old as long as you are still young. But when I see time's wrinkles on you, I will expect death to bring my days to an end.
For all the beauty that covers you is only the lovely clothing of my heart, which lives in your chest, as yours lives in mine. How, then, can I be older than you?
So, my love, take as much care of yourself as I will take of myself—not for my own sake, but for yours. I carry your heart and will guard it as carefully as a loving nurse protects her baby from harm.
Do not count on getting your heart back when mine is dead. You gave it to me, not for me to return it.