Devouring Time, blunt thou the Lion's paws,
And make the earth devour her own sweet brood;
Pluck the keen teeth from the fierce tiger's jaws,
And burn the long-lived phoenix in her blood;
Make glad and sorry seasons as thou fleet'st,
And do whate'er thou wilt, swift-footed Time,
To the wide world and all her fading sweets;
But I forbid thee one most heinous crime:
O! carve not with thy hours my love's fair brow,
Nor draw no lines there with thine antique pen;
Him in thy course untainted do allow
For beauty's pattern to succeeding men.
Yet, do thy worst old Time: despite thy wrong,
My love shall in my verse ever live young.
The word “Yet”, here used as a conjunction, represents the  volta  or “turn”, which changes the direction of the logic.  The speaker challenges Time to “do thy worst”; he is untroubled by its effect. The adjective “old” is flippant and disrespectful.
Sonnet 19