Where does the turn take place in "Sonnet XXX" by William Shakespeare?

Uncharacteristically, the turn in thought does not occur in Sonnet XXX until the final couplet.

Perhaps in keeping with the "remembrance of things past," the lines of this sonnet return again and again to ideas that "grieve at grievances foregone" in imitation of the persistence of sorrowful memories. The haunting of these memories is also connoted by the suddenness of the sibilant alliteration in the beginning—"sessions of sweet silent thought"—and the slower alliteration in lines 6 with the lingering sorrow of "death's dateless night" and line 9 and 10 with "grieve at grievances foregone" and "woe to woe." 

This poignant sonnet underscores the insight that people do not so much possess memories as their memories possess them. The speaker in this sonnet "pays" over and over for his memories of the loss of things he has desired, the time he has wasted, his defeats in love, and his failure to achieve many of his goals. Despite all his sorrows, the speaker need only dwell on the love of his "dear friend" and "all losses are restored and sorrows end." Indeed, the final couplet provides the resolution to the speaker's rue and grief:

For thy sweet love remembered such wealth brings
That then I scorn to change my state with kings.

The speaker's lover brings him such fulfillment that he forgets all his remembrances of "old woes," and he would not change places with any, even a king.