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.
In plain English
The speaker won't believe he's aging as long as the young man stays youthful—their lives are locked together. But when he sees time's damage in the young man's face, he'll know death is coming for him too.
The speaker claims the young man's beauty is actually just the outer shell of his own heart, which now lives in the young man's chest while the young man's heart lives in him. So how can one of them be older than the other?
The speaker urges the young man to be as careful with himself as the speaker is—not for his own sake, but for the young man's. The speaker holds the young man's heart like a nurse guarding a child, and warns: don't gamble with my heart once it's destroyed. You gave me yours to keep, not to return.
