Sonnet 22 is written to a beautiful young man whom Shakespeare loves and mentors (this is not a homosexual poem, in case you're wondering). It essentially says the following:
My mirror won't convince me that I'm old, as long as you are still young; but when I see that you are aging (have wrinkles), then I shall expect to die. Because you are utterly beautiful, you bring beauty to my heart (my soul), which lives deep within you, as yours does in me: How is it that I can be older than you? But be careful for yourself, just as I will be careful for you. Take care of your heart (your soul) just as I will keep yours, being careful of dangers and threats, just like a nurse protects a baby. Don't worry about your own heart when I die, because I will keep it safe even when I die.
The turning point in a sonnet is where the author puts a twist at the end. In some sonnets--the type with two quatrains and a sextet (two four line stanzas followed by a six-line stanza), the turning point is usually at the outset of the sextet. In this case, however, the form is three quatrains followed by the heroic couplet, and the couplet is usually the place to look for the "turn," or twist.
The turn here occurs at the final couplet: Most of the poem is about how each man has absorbed the beauty (the character, the personality, the soul) of the other into his own self so deeply that even death will not diminish it. He has hinted that he will probably die soon, but the young man should not be concerned, even then, because the young man's beauty will be preserved in Shakespeare's "heart" even in death.