Analysis of Sonnet 22: My glass shall not persuade me I am old

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 wary
As I not for myself, but for thee will,
Bearing thy heart, which I will keep so chary
As 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.