Sonnet 22 argues that swapping hearts makes two lovers the same age
The speaker of Sonnet 22 refuses to believe his mirror. As long as his beloved stays young, he cannot be old, because the two have exchanged hearts. It is a clever piece of lover's logic with a trap in the last line: a heart once given, he says, cannot be taken back.
The opening makes an audacious claim. The looking glass will never convince the speaker that he is old, so long as youth and the beloved share the same date. Only when he sees time's furrows cut into the younger face will he expect death to close his account. His own wrinkles, in other words, do not count; the calendar that matters is written on someone else.
The justification comes in the second quatrain. All the beauty covering the beloved is merely the fitting garment of the speaker's heart, which lives inside the beloved's chest just as the beloved's heart lives in his. If each carries the other's heart, how can one be older than the other? The rhetorical question makes the conceit sound airtight, even though the reader knows it is a charming evasion.
Since each now holds the other's heart, both must take care. The speaker promises to guard the heart in his keeping as carefully as a tender nurse protects a baby from harm, and asks the beloved to be equally wary for his sake. Then the couplet shuts the door: "Presume not on thy heart when mine is slain, / Thou gav'st me thine not to give back again." If the speaker's heart dies, the beloved's goes with it.
Worries about age had a real calendar behind them. Shakespeare appears to have retired to Stratford around 1613, at about 49, and died there three years later. His birthday is traditionally kept on 23 April, Saint George's Day, a date traced to the scholars William Oldys and George Steevens; the true date is unknown, but biographers like it because he died on 23 April 1616.
Source: Sonnet 22: My glass shall not persuade me I am old
