In Sonnet 19, the main theme is the poet is challenging the concept of Time while bargaining with the personified Time. While Time has a way of changing everything it touches, the poet is as if giving Time permission to destruct or destroy or mutate anything in its path. However, the poet asks Time to not touch his friend or loved one with the antique pen of Time:
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;
While the poet is trying to bargain with Time in reference to not touching his friend or loved one with old age and wrinkles, the poet has a recourse if Time should be so unkind. The poet will write his friend's youthful features into a poem which captures his friend as immortal. Yes, the poet will assure his friend's or love's youthfulness upon the pages of his poem. He assures that future generations shall witness his friend's forever beauty that comes with youthfulness.
Time cannot touch his friend's youtfhful beauty on the pages of the poem. The poet's friend's beauty will forever remain youthful on the pages of his eternal verse:
Yet, do thy worst, old Time: despite thy wrong,
My love shall in my verse ever live young.