Shakespeare's Sonnets: My glass shall not persuade me I am old

My glass shall not persuade me I am oldSo 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 theeIs 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 thy self so waryAs I, not for my self 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.